If it actually is pain related, then he will do it even if you are putting on a halter, as he accosciates(sp?) you putting something on his head with pain. So, rule out pain first, and then go to training.
For training, train him to ground tie, where he will NOT move, no matter! And start desentizing his head to every object possible. He may be head shy at the bridle from a bad experience before.
Start off by just putting the bridle OVER the halter, and having someone hold him. Once you can do that no problem, then do it while he is ground tied, then take off his halter, having someone hold onto him with a rope around his neck, and put the bridle on, once you can do this, move onto having no one hold him, just being completely ground tied while you take off the halter.
When I teach my horses to ground tie, after they mastered the basics, I ALWAYS teach them to ground tie with JUST a rope around their neck. Once they got that down, then it wouldnt be a problem if you slipped off his halter to put his bridle on. So dont just train ground typing with a halter on.