I would definitely take the ACT - S is a STEM kid and scored 2070 on SAT but 33 (no studying, 10th grade) and then 36 (2 weeks studying, 11th grade) on ACT. It is much easier for most STEM kids as the questions are more straightforward, and you avoid the marathon aspect of the SAT.
I’d pick up a copy of Chung’s ACT math book, take a bunch of practice tests and see how you do. Get a tutor if necessary.