For one, what type of bit are you using, and for two, what discipline are you riding? I don't know anything about you, your training, him and his training ect., so I don't want to make assumptions, but my guess is that he has never learned how to deal with pressure. Instead of pushing him into the bridle, and getting him to move up under himself properly which will naturally make him put his head down, and round up, you are instead just see sawing his mouth to get him to lower his head, and have only been using direct constant pressure as a means to stop him. You should half halt him when asking to canter, ect, instead of holding constant pressure, and you should ignore his head for a bit, and just push him forward from the rear, and get him going well under himself, then start adding light pressure to the bit, releasing once he gives even a little, then push and pressure again, until he gives a little, then release, and keep repeating that exercise. He'll learn that pressure is not a bad thing. That is just my personal opinion. I would honestly have a chiropractor look at him, and see if he maybe needs an adjustment, cause even if the saddle does fit, he could still be out of alignment, and that could cause him to be having bad reactions.