You know, minis seem to be really bad about this in general. I saw this at mini shows and I swore up and down that when
I got a mini
: I wasn't going to rudely shove his head into a bridle despite objections and that my horse was going to be a model citizen about taking his bit.
Then I got Kody. Ha!
Like you, my Arabs drop their heads and open their mouths waiting for the bit and no, they didn't do that when I got them. But Kody is a law unto himself and does his best to dodge the bridle any way he can even after almost two years. Okay, I found out that his first bit pinched him. But I fixed that and he relearns everything else in a heartbeat so you'd think he'd catch that it doesn't hurt anymore. He certainly stopped over-reacting when I'm driving him. I tried focusing on keeping the blinders from scraping his eyes and then on keeping his ears straight and un-squished and nothing helped. I had always thought it was caused by mini people not using the curtesy you would employ with a big horse who can raise their heads out of reach, but obviously sometimes it's the horse. Maybe it's the "coming in from above them" thing?
When I started taking formal dressage driving lessons a few weeks ago the instructor made me tighten up the cavesson a lot which meant I had to undo it to put the bridle on and I did notice a major improvement after that. And he's fine with the open bridle I bought him for siderein work. Who knows?
:
I agree with checking your guy's teeth thoroughly before you do anything else. Once discomfort is eliminated try some desensitization training. Ask him to open his mouth and then don't do anything. Do it without a bridle, then eventually pull a halter on after he opens and a few sessions later hold up the bridle and ask but don't put the bit in. Just get him to open and close and reward him for being calm even with the bridle hanging there in front of his face. Make sure you end the session here, don't try and force the issue. Do this when you are not planning on driving! LOL. You might want to try slipping some bailing twine into his mouth as a fake bit without a bridle at this point. Reward, reward, reward. Slip tasty things like molasses onto the bit the first time you try actually getting it in his mouth again. I think separating the bit and the bridle will help you determine just what his issue is. Kody doesn't like the umpire-mask-like driving bridle closing in on him but he doesn't really mind the bit. It took me ages to figure that out.
Your guy probably just has a complex by now and it will take slow and patient work to convince him he's over-reacting.
Leia