I think your son is overwhelmed. He needs down time, too, and rather than punishing him, I think you might be better served by finding a solution to the issue. His schedule sounds like pure drudgery to me.
Are there ways he can learn what he's supposed to learn from the homework without spending so much time on it? Can he dictate to you? Can he do every-other problem? Can you read to him, if that will speed things up? Have you talked to the teachers about how long the work takes? Can they make accommodations? Do they have suggestions? Can you get him a 504 that will require modifications to his homework?
One of my boys is 2E with dyslexia, and if I think he knows the material, I will help him finish his homework faster (dictating or whatever). It's allowed in his 504. So far he hasn't had this issue, but I would also read a chapter to him if that would help him access his reading more quickly.
A different accommodation I make for my DYS 10 (his twin brother), is that I let him stay home from school when he gets overwhelmed. But he's not overwhelmed by the work-- it's the slow pace that gets him down. Plus, he's in virtually every outside activity he can squeeze in and school interferes with his passions. (FLL, Semantics, sports, violin.) If he was homeschooled, we would be able to fit everything in more easily, but since he goes to school, we need to find an escape valve. He gets perfect grades, so I'm perfectly willing to help him find a balance by letting him stay home. Is that an option for your son? And maybe he could catch up on homework a little bit at the same time. (My son is home right now, in fact, and he has to finish his AOPS homework today.)
