Something else to consider: is he doing at least as well now as he was at his traditional school situation? (I know the answer to that, but it's useful to remember where you started.)
If he's doing better than he was, the next thing to ask is if there's reason to think there's some other option available to you now that's going to be better for him than homeschooling?
If not, then it might be easier to accept that you're doing everything right, but he's just a toughie.
Keep fine-tuning, of course, but that's part of the process of homeschooling for everyone.
