Based on my son's experience, I'd say have him take them both - early & often!
He had a 32 composite, with 30 or better in everything. He was triple-qualified, had his Rep's nomination, Eagle Scout, Boy's State, Varsity letters in wrestling and football... and got the TWE last April. So - now he's busting his tail at a four-year college, taking all the plebe classes, doing the Ranger Challenge w/ ROTC - and he just took the ACT for the 5th time on Saturday, looking to boost the math score just a little bit. His SAT (taken 3 times now) math score is 700, but he decided to focus on the ACT since he seems to do better on that test - your son might be just the opposite. Since they super-score, he's been able to boost all his scores here or there, just never all at once. He recently scored a 34 in math on one of the official practice tests, so there's always a chance to bump it up. Finally, the results can be SO variable depending on how well rested they are, what they eat for breakfast - why not find out if higher scores are possible? My son's 2nd SAT was taken the morning after a very tough football game and a late bus ride home - his scores in all three sections were exactly 50 points lower than his first attempt. If he's not absolutely certain that his current scores are the very best he can do, my advice would be to try again, and try harder. That's what the academies expect anyway!
Best of luck to your son.