Sit him down and ask.him to tell you exactly what joining up is about, where and how long his training courses are and where he could be working in the future. Its information that is easily available and stuff he should pretty much know.
Thing is, he might not even get in for some time, or not at all. His first training course isn't just 2 months and communication will be difficult as it's restricted and he will be very busy/absolutely knackered. The next course depends on what job he's going for. Once he's in his actual.job he could be doing shifts, weekends, other duties and isn't guaranteed time off when he wants. He may not even get his choice of base or area where he wants to work. He can't deploy at such a young age, either, but could later in his career.
I strongly recommend you follow your own career path. You don't want to be isolated or relying on him to be around or able to support you. You would be able to visit him,under certain circumstances, but you wouldn't be allowed to live with him on base for the foreseeable future as housing is strictly controlled.
Happy to take more questions.
Ex-RAF and ex-RAF Recruiter.