I think applying a second time shows commitment and tenacity. If he has college-level math and science, with decent grades, hopefully he is in a better position than last year. I wish him the best.
That said, as others have noted, it is possible that he COULD be denied an appointment again. At some point, a Plan B needs to be considered. I see posts on here of people (not the OP) who say "I will apply and apply" until I am accepted. In my personal opinion, I think two tries is about the max before it's time to move on with life (that's just my view, and others may disagree). My only advice is that, given the experience of a denial last year, your son have a solid plan B in place this year in case of second TWE.
Why does he want USMMA? Does he want to be an officer on a commercial ship? If so, in addition to applying again, might I suggest he consider the State Maritime Schools. Depending on where you live, you may qualify of in-state or in-region tuition even if you don't live in the exaxt state. For instance, a Maryland student gets in-state tuition at SUNY-Maritime.
If he wants the military college experience with a commission in the Armed Forces, maybe one of the SMCs should be considered (VMI, Citadel, Texas A&M, Virginia Tech, Norwich, etc.).
This is NOT to say that he won't be appointed. I truly hope he will. It's just, especially with applications the second time around, I like to stress the importance of a viable back-up to get you to your goal. It isn't giving up on a dream, it's re-focusing the dream.
Sounds to me like your son is doing everything he can to get admitted to USMMA on try number two. I hope he gets a BFE and starts at KP in the summer.