Sounds fantastic - I hope they have a wonderful time.
The last time I went Interrailing I was in my 60s and my DH was older. (There are older age discounts.) It's a great opportunity to see new places and meet new people and to polish/revive language skills. I still remember long conversation (in French, our only common tongue) with a retired Slovak teacher of about my age on a 5-hour journey across beautiful rural Slovakia. Educational in so many ways. And a charming young Ukrainian man who carried our luggage up steep flights of steps at our destination. NB Many stations - especially in rural areas - do not have escalators. Etc etc etc...
As others have said, PLEASE encourage him to get decent health/travel insurance. Not just the card - you need more. At his age it should be cheap.
Yes indeed to travel money card and to the cheapest roaming phone connection.
Yes indeed to travelling as light as possible, and planning to wash clothes en route. Many trains do not have much luggage space, and it's a real bore to have to carry too much around, anyway. Aim to always have one - ideally both - hands free. That means backpack and just perhaps one smaller bag-in-hand. For extras- eg picnic lunch - en route, a strong but roll-up-into-a-ball nylon bag is very helpful. I have one of these, but many many others are available: https://www.reisenthel.com/en/mini-maxi-shopper-AT-V-black-7003?c=199100000
Make sure all his tech stuff can be connected to Euro voltages to be recharged.. He'll need the right sort of adaptor - I'm sure he'll know more about this than me but it is very important.
In summer many/most long distance trains can be very, very crowded. For longer journeys, I'd strongly recommend getting seat reservations. Book in advance, for a few Euros. Some trains even insist on seat reservations. I expect your son knows this already but he can check the details of ALL trains across Europe here - a fabulous resource: https://www.oebb.at/en/fahrplan
Most European stations give out information rather differently from those in the UK. There will probably be a big central concourse indicator. And blurry announcements in two or three languages. But that's sometimes not enough. At big stations, each platform will usually have a display board showing all the trains scheduled to stop there and how they will be configured - ie where is first class, where are the standard coaches, and, if you have a reserved seat, where that will be etc etc . They will also show at which end of the - often very, very long - platform each train is scheduled to stop. (Some international trains have many coaches and it is sometimes not possible to walk, once on the trains, from one set of carriages to another.) Sometimes this sort of information does not matter; sometimes it does.
As previous poster has said, it might be handy to have access to the offline 'Bible', just in case there are connectivity problems en route: https://www.europeanrailtimetable.eu/
Station and public loos generally charge 50 centimes, so have some change at all times.
This website is ultra geeky, but possibly worth checking out: https://www.seat61.com/european-train-travel.htm Thankfully, DH and I have never had to use it but it might just possibly be useful one day.