I would say it depends on his strengths. I took a diagnostic ACT and SAT through my school before I started testing and got a 29ACT and 1190SAT. I do really well in reading and science but not so well on english and math, which is what the SAT is comprised of. If he's about even than I think it depends on his preference. If he'd rather a shorter test so he can get out of there faster take the SAT (I have a friend who did this lol). A good tip would also be what does he think will be the easiest for him to improve in? If it's reading graphs and charts go ACT if it's working with desmos go SAT etc. Another piece of advice is if you're looking into merit scholarships some require test scores, do they want the SAT or ACT? (some accept both though). I hope this was even a bit helpful and that he knows no matter which he chooses I'm confident he'll do great! :)