I've posted about this issue before but will again a it bears repeating for a new class of candidates. For those of you in competitive states and/or districts (which, quite honestly, is most of you), consider VERY carefully how to approach your noms if your MOCs require you to "choose" or "rank" the SAs in terms of preference.
If you would truly be happy at more than one SA, you may want to "split" your choices -- i.e., put USMA first with 1-2 MOCs and USNA first for the other 1-2 (adding up to your 3 noms). However, if one of the SAs is heavily your first choice, be wary of trying to "game the system."
A couple of years ago, I had a candidate put USMA down for 2 MOCs and USNA for 1. He got 2 noms to USMA and didn't even get an interview from the "USNA MOC." He asked me what he should do. I told him that he should go to USMA b/c, I hoped that since he'd listed it first for 2 MOCs, it was his first choice. Not sure it was but he was stuck, sad to say.
Another version of the scenario is that you pick X SA as your first/only choice. During the MOC interview, you're asked, "If we offer you a nom to Y SA, would you accept it?" It's hard to know how to answer b/c you don't know what the MOC nominating committee is thinking. They could be certain that they will NOT offer you a nom to your first choice but want to give you something, if you'll accept it. OR, they could be preparing to give yout the nom to your first choice but, if you'll take/be happy with a nom to your second choice, they'll give you that. Or, they could simply be trying to figure out what their options are . . . IOW, they are simply asking and it's not a "trick question." Remember, MOCs want to make the most number of constituents at least somewhat happy -- after all, they're politicians.
Thus, I think you must go into the process with a pretty good idea of what your first choice is and stick with that. You have to make a decision at some point along the way -- better to do it early.
I know some people say, "X SA is my first choice but I'd take Y SA if it's that or nothing." Unfortunately, what typically happens in that circumstance in a competitive area is that you end up with your second choice. Which is OK, if your heart is still in it. The "moral" of this story is that trying to "game the system" (and I mean that in a positive way) often backfires.
Finally, two ways "out" of this morass. The first is those fortunate enough to be able to obtain a Presidential nom, which will be good for all SAs. And, second, those who apply to USCGA, for which no noms are needed.
To answer your question re whether your son can get a nom to more than one SA from TX . . . it is probaly unlikely but not impossible. Remember what I said above, MOCs want to make the greatest number of constitutents happy. Thus, if a given MOC has 10 slots for each of 4 SAs (USMA, USAFA, USNA, USMMA), that MOC can make 40 candidates, friends, and families happy by giving each person only one nom. If that MOC gives one person two noms, that's only 39 happy people and one more unhappy set of friends and family.
The way duplicates typically happen in competitive states is: (1) the MOCs are simply willing to give noms to multiple SAs; (2) there are fewer than 10 qualified applicants in your district for one of the SAs to which you're applying (unlikely); or (3) you are so much more outstanding than the 11th most competitive person for the nom and the MOC thinks you have a much better shot of being appointed. I can't speak specifically for TX, but as noted, it's not common in competitive states with so many kids wanting to attend SAs and so few nom slots.