I’ve weighed in on this topic before:
If your son’s BS is well known to colleges and he has availed himself of the rigor there, that is all he needs to do. He will be at no disadvantage as I don’t believe colleges are looking for APs from BS applicants. If he is applying to colleges that give credit for or allow him to bypass some freshman classes so he can progress more quickly to classes that are of interest to him, then I think there can be value to taking AP exams.
FormerCK sat for a few AP exams that he self-studied for and did well. He ended up at a college that does not accept them, though.