It took about a month before my son actually finalized the decision on paper, but I think all along he had a preference for his #1 choice. I think they all do, even if they are not stating it openly.
Many apply to multiple academies, hoping that they can get into ONE. When their dream suddenly multiplies by being appointed to TWO, sometimes they may forget the real reasons, the first reasons, their "deep inside" reasons for applying in the first place.
My son compared his Summer Seminar vs AIM experience, the courses offered, the campus, the "feel" of the small school vs big school, his ability to play DIII varsity football (no chance to play DI - too small) but ultimately, the career choices AFTER graduation became the biggest factor.
I think it also came down to the humanitarian mission of the USCG: i.e. saving lives vs taking lives. Not that he has anything against the other services nor is he missing any "warrior" qualities, but I think he feels that the mission of the USCG fits more with his philosophy and life goals.
Your son is truly special, (spectacular in my opinion) and he is certainly in a very enviable position, having to decide between two winning choices.
A clue for you - when someone asks him about his college choices or college plans, how does he respond to that person? IMHO, whichever one he mentions first is his subconscious choice, whether he admits it or not.
Good luck, and enjoy your remaining time with him at home.