first time traveling in Europe

What are the ages of these two? What are they interested in - art, history, museums, architecture, food, nightlife, scenic spots, outdoor activities, meeting other young people, festivals? Any idea what their budget is going to be like? Big cities, small towns, countryside, combo of all three? Have they researched any particular countries, read any guide books? I'm sure they must have some idea of where they want to go and what they want to see.

A little more information would help - actually a LOT more information would help.

Make sure they know to plan 89 days, flying back on day 90, and not three months. Schengen rules.

Your son is (presumably) an adult. He will learn MUCH MUCH more doing this himself, but of course you can make suggestions. The first suggestion is for him and his friend to tell you their interests. I can tell you what I'd do now as 30something woman, or what I enjoyed as 20 year old bumming around Europe...but it's ultimately his interests that drive it.

First - get a map. A big map of Europe. Visualizing the geography will help immensely.

Next - interest. Foodies? Artists? Into military history? General travelers out to see the big sights? Nature lovers?

Key places many first timers like are London, Paris, Amsterdam, Prague, Venice, Florence, and Rome. I know there are great places in Germany and the Alps (Switzerland) but this is a black hole in my knowledge! Add in visits to smaller towns around whatever big cities chosen, and add some beach time somewhere (Spain, Greece?)

Rick Steves book Europe Through the Backdoor goes through a lot of basic info and key sights. I'd recommend it.

The UK isn't part of the Schengen, right? So if they are going for 90 days and starting in London, unless they head right to parts of Europe with the restrictions, they'll be OK.

they will be 22 - interested in beer, planning to stay in hostels, we have some relatives in Switzerland so they plan to visit them, what are the Schengen rules? thanks so much for the replies! I am just providing a starting point for them!

To be fair, I would direct a pair of guys in their early 20's to Lonely Planets Thorn tree forum. https://www.lonelyplanet.com/thorntree/forums/europe-western-europe
They cater more to the young backpacker crowd and have some very good pointers on both the budget, itineraries and on the schengen zone.

Regarding the Schengen rules,

the official answer from the EU is at:=
http://ec.europa.eu/dgs/home-affairs/what-we-do/policies/borders-and-visas/index_en.htm

and a very good comprehensive (some may say too comprehensive) answer from wikipedia is at:=
https://en.wikipedia.org/wiki/VisapolicyoftheSchengen_Area

I agree with Morten, Lonely Planet Thorntree and guidebooks will help.

Apologies for the Schengen comment, I thought the London reference was just in transit - rereading I see that there's a stay involved.

But to simply answer the question - the Schengen zone visa allows you to stay in any of the Schengen zone countries (most of Europe - some exceptions) for 90 days out of any 180 day period.

My dd and her friends are doing similar starting in mid april, they are both 20( well my is still 19, but will turn 20 in dublin..)

So for fun , beer partying etc they have of course included Berlin, Dublin, Amsterdam and a couple Greek islands( of course there is alot more to those places , but just saying they are destnations most young backpackers love and they will have a lot of fun there)

They are meeting us in Marjorrca end of May for some sun and good food( by then they will have been on their own living cheap for 7 weeks and cant wait for us to buy them some nice restaurant meals lol)
They are also including Paris , Florence and Rome,, for good food and amazing historical sites.

They wanted to visit Switzerland ( my dd has been there) but it was just too pricey for their budget( your son is lucky) .

They are taking some cheap inter europeon flights, i told them to book sooner rather then later and found some good flight/ prices, but they waffled on plans for awhile and as i said they would, some of flight prices went up, so tell boys to nail down an itinerary and book cheap flights sooner then later. Unless of course they are not on a tight budget.

If their first interest is beer as you say, then start with the big beer cities ...
Dublin, London, Brussels, Amsterdam, Prague, Budapest, Munich
Add in the other big cities along this route ... Paris, Berlin, Vienna
You could make a case for a week in each of these cities (maybe less in Brussels and Vienna) to enjoy the experience at a relaxed pace. Throw in 2 weeks each for Spain and Italy, and you've got yourself a 3 month plan