I didn't read prior posts but since you said its not a physical problem, I'd say its a training or riding problem.
A few things that worked for me:
- Sneak a treat. As the bit goes in, so does a yummy snack.
- Approach and retreat. Approach him, as his head shoots up, follow him, as best you can, if he drops his head even a little bit, retreat and rub him. It is a long process but, it will help. If you have a tall horse, like I did, grab a mounting block or step so that way you can stay with him.
- Go out on a day that you don't have enough time for a ride but enough time to mess around. Try the approach and retreat method and once he is bridled give him some grain or a few cookies and scratch his favorite itchy spots, unbridle him and then let him go play again.
The worst thing you can do is FORCE it in his mouth, it will only make things worse. I promise. I found that out the hard way.