WRT the USNA waitlist . . . sometimes they take a small number and some years no one comes off. They never take everyone and most years the number on the WL who ultimately receive appointments is very small. Not to burst anyone's bubble; just being realistic. As others have noted, it's all about yield. If more people decline than expected, then they will go to the WL. I would also note that, in the past few years, USNA has one of the highest yield rates in the country.
My advice would be to commit to USMA after the 15th and remove himself from consideration for USNA... that is what my son is going to do.
One word of advice in general (not directed at any specific poster). If your dream is USNA, you didn't get in and you're now accepting another SA, be VERY SURE it's what you want. I've endured more than my share of recriminations from candidates and their parents when the candidate attended another SA (usually USMA) because he/she didn't get into USNA figuring they'd learn to like it, it wouldn't be that bad, it was better than their other options (take your pick of reasons). They didn't like it and I heard about it. Some tried to transfer. One committed an honor offense in order to get kicked out. Not happy tales at all.
USMA is an absolutely terrific school -- as are all of the SAs. However, it isn't for everyone. Neither is USNA or USCGA or . . . If you don't really want to be there, don't "settle." Don't go hoping you'll learn to love it. Chances are that you won't. You could well be miserable.
Only go to a SA where you really want to be and enter a service in which you really want to serve.