There are so many math acceleration options now, that he can easily advance without leaving the high school and all the benefits for a teen. If he is math way accelerated, I was in math, then I would recommend an school like MIT or Caltech. But he also has to decide what he likes.

I have stated here before, but you may not have seen it, there was a kid at Hunter who finished a math and physics degree at Columbia and had his graduation a week before his high school graduation at Hunter. And then went on to that math center at NYU for grad or whatever level he was at. But it is good if a student is way advanced in math to still maintain his high school stuff in English, history, economics or whatever. What if he wants to apply his math to economics. It is a good idea to have these other subjects to explore his interests.