As you have seen, punishment, threats and other tools for control aren't working. They aren't going to work unless you break his spirit, which you may or may not wish to do. That is the purpose of those tools: to force or coerce a child into always succumbing to the will of the adults in his world. What he needs and wants must always take a back seat to the much-more-important needs and wants of the big people. Doesn't really matter if you agree with that, or want that to be what happens. It's what the tools of control say to children, and it's what they're for.
Little boy bodies (and human bodies in general) are supposed to move, be active and need to be so in order to circulate oxygen to the brain -- exercise is the best tool to aid concentration. After a day of being coerced or forced to sit still doing what someone else thinks is important, a protracted period of free play and big-muscle movement will vastly improve his ability to focus on the task of once again sitting still and doing what someone else thinks is important. Meaning: insist he play outside at recess, lunch, before and after school so he gets lots of running-around time, and don't even talk about homework until well after dinner when he's getting tired enough to sit still again.
It has been clearly demonstrated that people learn best when they get to sleep on today's lessons, so perhaps starting the day with the rest of the homework is a better plan than trying to enforce 'more' late in the day.
Alternatively, you could read The Homework Myth, by Alfie Kohn, for an in-depth, evidence-based argument about how damaging and useless homework is. I have known a number of parents who told their children's teachers that they can do what they want with the 6 hours they have the children, but after that, it's 'family time' which will not be polluted by work that was not completed in the 6 hours they already had.