In addition to his teeth, I would check saddle fit, as well as make sure that the bit you are using fits in his mouth, and isn't pinching, or jabbing him in the roof of his mouth. If you can, I'd also have a chiropractor come out and adjust him. If all those check out, then I'd see how you are riding him. Are you messing with the bit a lot, are you accidentally thumping down in the saddle, or being overly bouncy when he's moving. Oh, and also check his ears. I had a horse once who had really bad gnats in her ears, and because we didn't shave her ears, we didn't know until she really started to shake and throw her head around, and started getting sensitive when we would bridle her. Once we realized it, we got a fly mask with ears that we kept on year round, and made sure to apply fly spray to her ears as well when we were riding. Hopefully you can get to the root of the problem. I applaud you for not immediately going for tying his head down, and instead asking for help and ideas.