OK J.! I am actually a middle school teacher (Or was before my baby) so I have seen both sides of school work. I had students who raced to do their homework and did poorly, students who were meticulous and those who just refused to do their homework. With those who did their homework poorly I would take away their breaks and make them re-do it while they had to watch their friends enjoying themselves. They say EVERY break until it was done correctly too. Now, in a public school I know that a teacher cannot do that. However, here is the LOOPHOLE! (I am the queen of those!) YOU can write a note to your teacher so that it's on file that it is YOUR decision and not the schools stating that YOU want him to give up his breaks at school to re-do his schoolwork. Usually the counselors/office will be more than happy to help you if it means their academic standing will go up with more students completing their work and doing better on tests. If there is no place for him to sit in the school yard, then he can be sent to the office, books and papers in hand. It's kind of an embarrassment technique and it never failed me.
On the other hand, for the students who just plain refuse to do their work, there is always retention. Don't spring it on him though. He has to be allowed to make his own decision because partly this is what he is rebelling against. He is being told what to do and when to do it not only at home, but also at school and he's trying to exert some independence. Albeit it's misguided, but that's where this is stemming from. So give him all of the facts. Students who do well... go on to the next level. Those who don't stay behind and since this is more than likely his last year at his current school he will also lose a great deal of his friends who will be going on to a new school. Does he want to face that? This is another subliminal embarrassment technique. If he chooses to continue to not do his work then you must back up WHATEVER you told him. Some schools still secretly (or blatantly) continue social promotion or promotion based on test results. You as the parent still have a say as to whether or not your child continues on, especially if you have a report card stating he doesn't know the material from 6th grade.
Good luck to you!!! If you're in the San Diego area I'm available to tutor if he makes up his mind to try to get caught up.