As a solo traveler myself, and a daily contributor to a solo travel forum, I see a big difference in the skills of this generation and mine. There is a herding mentality that I didn't experience - though not everyone is that way. It just seems that being confident in making a decision and actioning it all by yourself seems harder now.
When a new solo traveler asks for help with a global trip, we often recommend doing something close to home. Planning a weekend away without the help of his mates will go along way to preparing him for solo travel, or helping with planning his trip with you. As mentioned above, a major US city would give him closer to reality exposure. Maybe his folks would foot that bill, but if he isn't 18 at the time, it becomes more complicated for lodgings - keep that in mind.
I would also buy him a guide book - I like the Lonely Planet Europe on a Shoe string. Once he reads the front and back, he will know what needs to be decided upon or when on the road, what to take not of - and will cover all future travel for bullets to fill in regardless of country. It might also give him a feel of what country may appeal to him for this trip. Now, some might find it overwhelming as a starting point and if that might be the case, then buy him the country guide - please, not just Rick's.
As said, YouTube has any number of travel videos, and also airport, subway, train videos.
As you have a year for this, I would be tempted to set him tasks that you review together after he has had some time to work on it
-Decide on a destination (perhaps to start, find out what he wants to see, and if there is a favourite type of art or artist he likes. If too overwhelming, explain what cities carry the most of that art then let him think about it and decide).
- how to get there (google flights or https://matrix.itasoftware.com/ for wider range of flight options and non partisan planning). When reviewing this, discuss the pros and cons of using various websites. airline site vs a 3rd party site.
- transport from airport and in the city (you tube)
- good location for hotel (review safety, ease of access to transport, or walkability to sites)
- what to see?
- currency, electrical currency (perhaps discuss how to pay and access an atm)
- together, you'll want to decide how to communicate with each other if he goes off on his own
- look at seat61 and rome2rio for future tools
All that being said, is your relationship close and relaxed enough for him to give you a straight answer if you ask him....'do you want to learn how to plan travel, or just have someone else do it?' Travel planning doesn't interest everyone and as much as you want to guide him, he may not want to do it....