I used to be a residential hall director and hired RAs for my staff. The process varies from school to school, but there are some similarities across schools.
My school didn’t prefer upperclassmen over sophomores. Sophomores were welcome if they were mature and we thought they could handle the work. In fact, we often put sophomores in our first-year and sophomore halls, although we did try to avoid putting them in mainly junior and senior halls. On my area’s staff of 35 I’d say during a given year, around 1/4 to 1/3 of them were sophomores.
What is really important is some kind of transferable role or skills that the student can use to inform the RA role. We used those for two reasons. One, having held a similar role in the past was a good indicator that someone was actually interested in being an RA and wasn’t just applying for the free housing. Two, having that background is tremendously useful when actually working as an RA. There were lots of similar positions we saw and hired for: some people were RAs or prefects at their boarding schools, but most people don’t have that kind of experience. What we also saw often were mentors, tutors, camp counselors, peer mediators (this is a really, really great background to have!), crisis hotline volunteers, hospital volunteers, Big Brothers/Big Sisters, that kind of thing. Any kind of experience like that would be a huge help to an RA application. (One of my favorite applicants was a freshman - but he had taken two years off school to do a Mormon mission abroad. He talked about how his experiences as a missionary were useful and transferable to the RA position. He ended up getting hired.)
I will also add this: My school had an ROTC program, and we were always careful when considering applicants from ROTC. It’s not that there’s anything wrong with it - it usually indicates a level of discipline and maturity that other students don’t have. But ROTC is very time-consuming, and some students have trouble balancing the two. We did the same kind of mental calculus for any of the other activities on campus that were known to be particularly time-consuming -any kind of student government position, a couple of activities we knew were super-crazy, or any student who seemed overly involved in a lot of things. Students who had those listed on their application either had to have some very compelling, well-thought out plan for managing them OR indicate that they were planning on dropping an activity or two (or more) to accommodate the RA position. (However, that doesn’t mean you can’t be involved on campus as an RA, and we liked our RAs to be visible on campus. Our RAs usually tended to do 2-3 other things, and a lot of them were athletes. You just have to strike a balance.)
I know the Reserves are not the same, but a lot of people do not know what the Reserves entails so your son should be ready with an explanation for how much/little time it takes up and how easy it is to schedule RA responsibilities around the Reserves. FWIW, one of my favorite RAs ever was a National Guardsman while he was in school (AND he was in a fraternity AND he was the president of his Greek council AND he had other leadership and activities on campus AND his grades were excellent. He’s doing a PhD at Berkeley now. AND he was a delightful human being).
Also important: get to know the current RAs and attend events in your own building! We absolutely took that into account when evaluating people. I trusted my RAs and relied on them heavily to bring good people to my attention. No RA wants a dead weight on their team, so they only recommended good people to me. And they play a big role in the process - senior RAs often conduct the interviews along with RDs and more junior RAs help with the process in other ways. One of my other favorite RAs (aw man, I just loved my RAs in general) was really involved with residential life before he became an RA and helped his floor RA coordinate and plan activities. Understandably, RAs also often want to work with their friends, so forming a good (genuine) rapport with your RA or an RA in general is a good idea. Frankly, getting to know your RA(s) is a lot more important and helpful than hobnobbing with the Dean. Our Dean was not at all involved in the selection process.