Without knowing the particulars of what he's studying in what his particular school calls 7th-grade math, I suggest that the best course of action on the math front is to do nothing.
1) Based on the previous history, it looks like your DS has plenty to deal with on the executive function front. He could very well benefit from using this year as a personal growth year, with executive function as his focus, because harder material is not far ahead.
2) This is 7th grade, not 2nd. The material is more difficult to fill in when gaps occur. And the further into the school year he gets, the more material he would have missed in the new class.
My DD12 experienced 7th grade math last year through a cross-country move, nearly halfway through the school year, to a new school with a very ineffective teacher. Then she began 8th grade in yet another new school, only to be transferred once more three weeks later. While her experience wouldn't be as significant as skipping an entire year, it did leave her behind the curve in 8th grade math (all other subjects are fine). And while her executive function is excellent, she is investing the time after school with a really great teacher who is committed to helping DD excel, and DD's grades are plenty acceptable as a result, my DD does share your DS's trait of perfectionism. DD's get-it-done attitude does get frequently interrupted by howls of self-doubt. I'm constantly having to counter-message, because "I'm bad at math" is not a successful identity trait for a future robotics engineer.
