If he has no real desire to attend WP, I would never put that as #1 choice. Gaming the NOM process has back-fired in the past for some.
My DS would be more than thrilled to attend USMA but without a doubt USNA is his 1st choice.
My son had the same situation, but had a nomination in hand for USMA, USNA and USAFA from congressman, and another for USMA with one senator, and although USMA was his first choice, he asked and was allowed to switch his first choice with the second senator to USNA, figuring he would increase his odds there since he already had two at USMA (actually he has three at USMA, because the first senator had two slots open and he got two from her). There is nothing wrong with putting yourself in the best position to get a spot somewhere.
In your son's case for sure I would rather open the doors for an appointment at two accademies, rather that adding one slot at USNA, and I am not saying that because I'm a West Point dad. If your son gets a nom for both academies, but does not win that slot, he will still be able to compete for an appointment off the NWL. If he gets noms to both schools he competes for the slot at each, as well as the NWL at each, so effectively four pools. And while some may challenge this, regardless of where, if your son gets an appointment to any academy and attends, he will not likely ever regret not going to the other academy. Indeed, he will almost certainly count his blessings that he landed where he landed. Kids at that age, moreover, have a lot to experience, and their views on a lot of things will change. My son thought for sure he wanted to branch aviation when he started at WP, for example, but there are four things now which are higher on his list. Just the advantage of perspective.
I should have caveated my statement. In certain parts of the country, obtaining a nom to one SA is much harder than obtaining a nom to a different SA. You should be able to figure that out -- or ask your BGO/USMA equivalent. In such a case, the MOC is looking for ways to nominate candidates to the "less desirable" (in that area) SA and still make those candidates happy. Thus, if it's a very competitive USNA area and a candidate says he/she would be happy with a nom to USMA -- whereas another candidate says USNA only -- guess who's more likely to get the USNA nom. As noted, if USMA is still a very desirable choice or if USMA is hyper-competitive in your area, then not as much of an issue.