If he's going to Va Tech then he will have the choice of Army, Air Force, or Navy/Marine ROTC, so that's choice one. How important is a scholarship? If it's very important, then a consideration will be his major, because that might affect likelihood of a scholarship depending on the service. Also, of course, is his interest in pursuing a particular military specialty. If he wants to be a fixed-wing pilot (for example), then Army probably isn't a good choice. OTOH, if he wants to fly helos, then Army might be a really good choice. Or if he wants to be an infantry officer, well, it's Army or Marines. So those are all questions to answer.
Once he's done that, then I imagine (and I base this on how things work at Texas A&M, which is also a Senior Military College with a Corps of Cadets within the student body) he just needs to inform that Corps at VT that he wants to join, and they'll take it from there, and part of that enrolling process will be to sign up for whichever ROTC he wants to join.
One other suggestion. If he really doesn't have much idea what he wants to do other than serve, then I'd suggest going with the service that has the most diverse set of options. I tend to think that's the Army, but others might disagree. In any event, I'd suggest he look at the array of things all the services all do and consider what appeals to him.
Hope that helps.