Which test to take or if he should take both tests is determined by what colleges he is interested in. Generally even the top schools in the Midwest are looking for high ACT scores, and writing the essay is a plus. The big schools on the coasts generally want the SAT. (Its essay is not optional.)
However, to maximize your investment you MUST investigate each individual school's expectations of its incoming freshman class. If both tests are indicated on the basis of what I've just said, then my counsel is to prep and sit for the SAT first, then the ACT.
You may even want to schedule one of each test before investing in paid test prep. It's enormously helpful to me to have that baseline already drawn.