Again, even as a junior, he has no idea where he will be going to college, so the credit/placement may matter, Without knowing the specifics,it’s hard to say more, because it depends. Is APHG exam worth taking? Probably not, because most colleges give no credit. IS AP Calc BC worth taking? Yes, because credit/placement is almost universal. But if the AP is in a subject he’snot doing well in, then he likely would not perform well enough on the exam to get credit. So like I said, “It depends.”
Will there be an expectation that a student take the exam when the school offers no APs? Of course not. Will college AOs look at AP exams to validate grades from a BS? No, but that would be a valid reason for a student from East Podunk High.
But I’m not understanding the part about rigorous prep school with big ECs. We all have/had that. If we wanted the credit, we sucked it up and took the exam. It’s also unclear who’s driving the decision; you seem gung-ho that he not take it, but what does he think?