In my opinion this is a personal choice and something only your son can answer. Only you guys know what schools are recruiting him and where he stands. With that being said, as a recruited athlete, if he is offered prep and West Point is his goal, take it.
I was a recruited athlete also and only applied to USNA. I did not apply for ROTC or another service academy. It did come up in my MOC interview (no BGO interview as a blue chip basketball player, we weren't allowed to talk to them). They asked about the other Service Academies and I told them point blank I wanted to be a Marine and USNA allowed me to pursue my passions of basketball, academics and becoming a Marine. I told them that I did not apply for ROTC, because as a Division I athlete (especially as a basketball player that takes up so much of the school year) it is not necessarily compatible or offered at other schools I was being recruited by (yes it can be done, but extremely difficult and IMPO cut myself short in one or the other as the Coach/MOI wouldn't necesarily agree on what my top priority would be). I did tell them that I had done research and had planned to purse a Marine Corps commission upon graduation. I explained that route and I think it showed them I was committed to the idea of service as my goal and not just to attend USNA as a jock, but I took being commissioned seriously. I think they accepted this fact as I had done by research and was serious about being a Midshipman and future Marine Officer.
To not apply anywhere else is very risky. He could be medically disqualified, get injured, or a million other things this next year. Having a Plan B is in his best interest. I am assuming your son is being pursued by other schools? Applications as a recruited athlete at other schools is very different and the coaches can advise on those. I would apply to any school he is seriously considering attending. As to ROTC applications that is his decision to make and how it would impact his sport, academics, etc. But he does need to be prepared to answer the question at his MOC interview.