First time traveler

From personal experience, he might want to start out doing the Camino Frances, as he would meet so many people, many of his own age and though he would start out solo, he wouldn't be alone. An organized group is not needed. By the time he finishes, he would be confident enough to go anywhere else in Europe he wanted.

I am not able to answer your question regarding joining a travelers group.
However, I have traveled over about 80% of Europe over the past 43 years.

I do remember in my younger years traveling on a low budget and staying in some very cheap and very borderline places.

Before the internet, guidebooks were a traveler's best tool for touring, transport and lodging.

I did find some stuff on the internet that might help.
https://www.tourradar.com/hg/backpacking-groups

I always felt relatively safe in Europe, compared to the USA, but Europe has changed quite a bit over the decades. I believe that a backpacker traveling alone needs to take more care with where they travel. I lived in Germany for four years and still have Germany friends that warn me that going out at night in that country is much more a concern, especially concerning where you go in its cities.

Also, I know that many more tourists are going to Europe and that visiting key sites these days usually require booking in advance.

Hi inkatshands -
I think you're just looking for some help indentifying the best resources for this young person? I can see where that could be a bit of a challenge for the novice backpacker.

Some issues here are that the RS forums are populated largely by non-backpackers, and probably not the age of your friend's son so I'm not sure how many relevent responses there would be if posted in the General Europe forum where the question really should go. As well, the cost of a 90-day trip is likely beyond the budget of many younger folks so he might need to string multiple trips together with multiple groups. He may even want to look at mixing his solo time with some package tours specifically for persons of active younger age groups.

In individual locations, hostels would be his friend both for price and some companionship; many offer group tours and activities. For instance, this highly-rated hostel in Rome that offers different levels of accommodation (dorms to private rooms) and shared activities from social walks to game nights.
https://www.the-beehive.com/activities

Hostelworld (good resource locating hostels + reading up on amenities and ratings + bookings) also has a function which allows clientele to connect with others. Might be a great place to start, although he's going to need to have a idea WHERE he wants to go. Have him take a look?

https://www.hostelworld.com

Facebook groups are also said to be useful, and he might look at Couchsurfing and Trip Advisor's solo traveler forum.
https://www.couchsurfing.com
https://www.tripadvisor.com/ShowForum-g1-i12357-Solo_Travel.html

Hopefully some others have additional resources to steer him to.
For a few companies with group tours of different lengths aimed specifically at younger people:
https://www.gadventures.com/about-us/
https://www.contiki.com/en-us
https://www.intrepidtravel.com/us/18-35s

Just a start? Lastly. 90 days is a long time for the maiden voyage. He may want to consider getting his feet wet with a less ambitious plan and then weighing what appealed/worked and what didn't to plan the next trip? Might be less overwhelming than planning a "Grand Tour" too. And I'll absolutely agree that the young man should do the homework himself so he learns the fine points of trip planning but I figure you know that. :O)

Every travel plan starts with an interest or wish. What are his interests or wises for a Europe trip?

If the boy is a student, he could look for trips organized through the school, church, or associated organizations. There are a lot of logistical issues for which it helps to have an organized group doing the planning. In my mind, the "how" is more critical than the "where" at a young age.

90 days. Red alert! The first thing he needs to understand is the Shengen rules. Lots of info can be found on this forum. Also, 90 days (or even 89) i a long time for a neophyte.
Many years ago, my younger daughter spent about a month traveling alone in Europe, mostly staying in hostels, after a year of studying in France. She had had lots of experiences of shorter trips and was a fluent French speaker. She still experienced some glitches, like getting money stolen.
My advice for your friend is:
1) Shorten your trip
2) Find a travel buddy for at least part of the time, or join a group
3) Start now learning about the nuts and bolts of European travel. I think Rick Steves may have a book on the subject.

"Was hoping you might be able to suggest a direction for him to begin his plan?"

I think your request for direction on the forum here is perfectly legitimate. I think you have received some good, specific advice so far.

What I find odd is that someone would misinterpret this request as a call to research the young man's 90-day trip for him, which clearly is not what you asked for. He of course will figure out all of that in his own way.

"...90 days but maybe with a small group who are looking to do the same just maybe not alone for the first time out?"

90 days of pure uninterrupted travel is a lot... I would like to toss out the idea of joining a volunteer work group or work camp for part of the 90-day stay. Your friend's son may or may not meet someone with shared interests to travel with afterward. But he will surely make contacts and develop relationships with the other young adult volunteers from the local area and from different countries in a setting like this. I have known others who took part in such camps - they did develop friendships and gathered useful travel tips, but beyond that, they also enjoyed being of service to others in the company of others. For a couple of weeks, some of them restored/groomed hiking trails, some cared for the elderly... not necessarily easy work... but a work setting like this is a unique travel opportunity, a chance not just to converse with shopkeepers and waiters, but to get to know Europeans and others in a more personal way than you can just by sightseeing.

I have no specific recommendations for finding the right fit. This organization is just one of several examples:
https://ibg-workcamps.org/en

I agree with the suggestion to start with university resources if this young man is at university now. For example, has he done or will be doing a semester abroad to jump start his adventure or are there professors going to Europe next summer for academic purposes that this young man can join as unpaid staff.

Maybe the RS book, On the hippie trail, may interest him. Times have certainly changed, but the same urge remains.

Has he decided yet? Is he committed not go it alone and has opted for being part of small group ?

I went solo the first time in the summer of 1971 for 84 days, exactly 12 weeks , a real green horn backpacker at 21 and all that, etc. While I agree that Europe has indeed changed quite a bit over the decades, I still go solo unless it happens to be partially with family members. I don't go with friends, never have, rendezvousing with them in cities is one thing but going from place to place is quite another.

Bottom line...I would suggest detailed planning with regard to setting priorities, places and sites to be visited, the logistics, a compatible travel style, ie knowing oneself, etc.

IF he speaks passable French, a great set of guidebooks would be Guide Routard. Designed for his French speaking demographic.

I'm not young but I've spent nearly 100 nights in hostels with mostly young people in Europe, so I have an idea or two about this. I've not heard of or encountered any small groups who are traveling together. Couples yes, and not necessarily couples who are linked romantically, but haven't been a "group."

What about going it alone, but having an escape plan after 10 days, maybe 21 days, 30 days if he is not liking it, or not adapting quickly enough?

A good plan would be to go to a city where he wants to stay for a week or even more, booking a hostel. If he is outgoing enough he's going to meet people there, and that will influence how he thinks about this whole adventure. Maybe he will learn the art of finding someone in a hostel who wants to continue to the next city with him.

As someone mentioned already, the best place to research lodging is hostelworld.com, reading the reviews before choosing and also looking for hostels that have either a lot of hostel-organized activities such as tours or pub crawls, or a fairly large lounge area.

Here are the two best resources I know of for more about backpacking. Both are run by young men who have a lot of European travel experience to share: thesavvybacker.com and nomadicmatt.com.

Is he over 18? If not, he may run into additional hurdles...

If alone, do the Caminhos de Fátima.

Rick’s son, Andy Steves, runs trips for younger travelers. Maybe check out his tours to give your son a jumping off point.
You could call the office for his info.

I thought about Andy's tours but last I looked he doesn't appear to be running them anymore. I just checked again and that still appears to be the case although it looks like he can help customize a plan. His WSA site (Weekend Adventures) is incompletely inactive. There does look to be a 2 hr Budapest group walking tour but that's the only one I see.

He ( the friend's son) could look at the travel tips, I guess:
https://andysteves.com

Has he ever done any domestic travel on his own?

What he is attempting is a bit like having your first swim lesson at Ka Lae, which may be a less than an optimal choice.

https://duckduckgo.com/?q=south+point+hawaii&t=newext&atb=v395-1&iax=images&ia=images&iai=https%3A%2F%2Fimages.theoutbound.com%2Fcontents%2F117537%2Fassets%2F1490463071311%3Fw%3D1200%26h%3D630%26fit%3Dcrop%26auto%3Dformat%26s%3Df7ff7e74921a6113af18bc3e4bceeae6

Did a search for 'europe travel for young groups' and got this:

https://duckduckgo.com/?q=europe+travel+for+young+groups&t=newext&atb=v395-1&ia=web

It might help

Mark Wolters is a university professor who takes students on trips to Europe. His YouTube channel (https://www.youtube.com/@woltersworld/featured) is geared towards college-age travelers, and he has loads of videos on budgeting, travel skills, how to ride the trains, as well as destination videos. All the videos are short (10mins or so on average).