You set a bar that is too high for him, and

 

that bar keeps moving.

He has no confidence that he can clear that bar, which he doesn't even know its fixed position because that bar is not in his head but yours.

To seek the comfort of certainty, he unconsciously distracts his attention to something that is tangible and under his control, like taking steps.

Say to him: I don't care what grades you get, or what college you go to, as long as you're making your best effort. And really mean it.

Get the perplexors from mindware.com.  Have him do two questions a day.  They are not difficult but condition him to think.