Getting back to the original question, have your son take the ACT. No school except BYU (so I hear) requires one test over the other; use the better test.
Prep work is important though, especially if your son is ahead of his class in math like mine was. He really benefitted from reviewing algebra/trig/geometry before taking the ACT.
Most importantly, don't worry (and tell your son not to worry) about everything hinging on the test score. My daughter had an OK GPA, so-so SAT, and a fairly good ACT score. But because she had taken 50 credits of college courses and done well, she got accepted to top universities that her friends with better GPAs and higher test scores did not. In fact, one friend with a 2000+ SAT and 4.0 GPA was told by an Ivy that "he had not challenged himself", and that he would have to attend a prep school for a year to demonstrate he could operate at a collegiate level. Most universities these days want students who have demonstrated they can handle college-level curriculum. This means AP or actual college classes.
Best of luck.