AP Calc AB is equivalent to 1st semester college calc and AP Calc BC is equivalent to 1st and 2nd semester college calc, so the pace is approximately twice as fast in Calc BC. I would consider the math score on any prior ACT/SAT tests as a indicator of probable success in AP calc, the higher score the better and could be a data point in your decision. Also, how were his test grades in prior high school math, not counting homework points, extra credit and so on, as these are notorious for high school grade inflation, compared to test scores that measure knowledge?
At our high school you take one or the other, however at some schools they require you take AB one year and BC the next. It varies.
Admissions at all colleges will value Calc BC much more than Calc AB, because so many less kids challenge themselves with BC and having earned an A in both semesters of Calc BC and a 5 on the AP exam is a clear verified indicator of a students readiness for college math intensive majors such as engineering.
An "A"grade in Calc AB will probably be more highly valued than a "C" grade in Calc BC. Best advice, ask his last math teacher what they recommend, they just spent a year with him and should have a good idea of his probability of success in Calculus.
Another consideration is since he is a senior, most colleges will not see the grades he obtains senior year until after he is admitted, however they may see the first semester grade and it could become important especially if he is wait-listed and they ask for first semester senior year grades. So, his first semester grade will not be a factor if you apply early admission since it has not been given yet, but could become a benefit/concern if he applies regular decision or is wait-listed from EA. The AP test given in mid-May will almost certainly have no impact on admissions decisions as it is too late in the cycle for consideration, but could at some colleges give him credit. At USCGA they note the AP score, but give their own math test during SWAB summer to determine placement.
At most colleges of the 3-semester engineering Calc sequence, Calc 2 is considered the most difficult class. So exposure to some Calc 2 topics in High school Calc BC could be helpful.
Whatever you decide, the absence of any Calculus on a transcript will almost always be a negative for a STEM college, so he should take one of the two calc classes. A good knowledge of Calc also helps a lot in college physics.
My understanding is Calc and Chem are two of the most challenging 4/c classes at the academy and can even result in summer school (missing afloat training) significantly impacting their schedule and in most cases dragging down their GPA in other courses as well. For that reason, I cannot overemphasize the importance of encouraging HS students interested in STEM or an academy to take at least AP Calc, AP Chem and AP Physics as the rigor in those classes will greatly benefit their first year of college. In a perfect world, he would take the 3 APs and reguardless of the grade, the experience of taking them will help him in college; however, if he bombs the grades since we are talking about the warped world of college admissions, the low grades even in challenging courses may be held against him and he may be penalized for challenging himself.
Lastly, however busy he may think he is now, it will not come close to the demands of a military academy. That said, don't forget to balance being a kid and the mental health concerns of over scheduling young minds with academics.
Sorry for the long post, lots to consider.
Good Luck!