Join Date: Jan 2009
Location: Tri-State (IA, WI, IL) area
My two QH mares had that problem when I got them - still do, a little bit, but it's a work in progress.
Whenever you are doing anything around him, play with his mouth. A lot. Get him reeeeeal used to having hands on, in, and around his mouth! (Be careful not to get bit though.) Hand-feeding helped my girls a lot. Also, work with going through the motions of getting him to open his mouth -- that is, stick your finger/thumb in his mouth (in that gap behind his front teeth and in front of his molars) and playing with his tongue/rubbing the palate until he opens his mouth. Then take your hand out and reward. Repeat.
When you go to bridle him, don't just jam that bit in his mouth and off you go. Take it slow, as if he were totally new to the bridle. Lay the bridle over his face and reward him for letting it sit there; then use that trick to open his mouth, slide the bit in, let it stay a bit until he settles, then take it back out and reward him.
If he's especially tall like my boy is, and has learned that neat little trick where he puts his head up so high that you can't reach to put the bit in or work on his mouth, work on teaching him to put his head down -- pinch that bony ridge between his ears until he drops his head (even a little), release, then reward and repeat until he'll keep his head calmly on the ground.
Also, it couldn't hurt to have a vet check if he needs his teeth floated, make sure his bridle isn't too tight, try switching to a different bit (dependent upon the type of bit you're using, it could be too harsh and hurting his mouth) or making sure he's not getting his mouth yanked on a lot.