ACT is a very viable alternative. The Guidance Councilor at our school said kids from our school typically score 5-10% better on the ACT - with some kids doing as much as 25% better.
SAT is more abstract than the ACT is how my daughter who took both described it. She did substantially better on the ACT.
Easy enough to find out - the ACT prep books have full tests in them - have your son take a practice test under the same conditions. If he does better, there is an ACT test being given this weekend - he'd have to go standby.
For what it is worth, I am not aware of a school that only takes the SAT - everywhere I know of takes both.