I am working with Brooks, as many of you know. At this point he's got a lot of the basics down: he leads well, he responds to pressure (on his head too), he can be tacked up and tied up and he will stand, he can lunge, I've been on him a few times, etc.

However, he's got some issues with his head/mouth, and I don't know his background so I'm not entirely sure of the cause of the issues, I'm just dealing with them now. I can rule out any sort of pain though as he has been thoroughly checked over by a vet and he's perfectly healthy.

So, basically he won't let me put anything in his mouth. He starts to back up and try to rear and tosses his head way up (which, I know isn't much of an issue for some people, but I'm super short). Basically he throws a fit. And I've tried to calm him down, get him to bring his head down, breathe and relax, but then as soon as I try to do whatever I'm doing, he goes crazy again. This is an issue with things like dewormer and also bits.

Right now I'm using a side pull on him, but it's not working as well as I would like, so I think it's probably time to upgrade to a snaffle.

Which brings me to his other issue: his head. When he's on a line, or standing, or if you have any sort of firm hold on his halter/lead at all he will start tossing his head around. He throws his head constantly. It's almost like he gets agitated by being held/tied up? Or that's how it seems to me. Although, I will note that if you've got a loose hold on him, he's a perfect angel. However, keeping him on a loose line is not always an option, so this issue needs to be addressed.

Anyway, I'm just looking for suggestions on things I can try with him, or even root causes that may be the issue. Now that it's warm again I want to get serious about solving this problem. but I'm kind of at a loss at the moment (especially since I don't know WHY), so just looking for something somewhere which will make me go "ah ha!".

So, having worked with some pretty weird horses myself, you may NEVER know the reason why, and for a while it might just be a bit of damage control lol.  He could have been tied and backed up and panicked.  He could have been beaten in the head.  I used to ride a retired grand prix horse (that was imported from germany a decade ago) that flinched his eyes and mouth when you went to put his bridle on because he had his tongue shocked to force him to accept the bit.

Have you tried something as simple as just putting your finger in the side of the horse's mouth and then before he has a chance to over react, have him lower his head and reward him?  Until he gets the routine down that "when something is put in my mouth I accept it and put my head down and then I get a treat".  And then just kind of up the anty to an empty syringe, or syringe with apple sauce partially squeezed out of it; and then again head down, treat.  I had a mare that had gotten a tooth infection before I purchased her and must have been forced with large amounts of SMZs so whenever I wanted to give her anything (wormer, bute, etc) it took two people, a stud chain and I had to back her into a corner (and I was just gald she was a pony) but I just hated that it was so agressive to get something simple done.  I also had to pick my bit wisely; she was always ridden in a happy mouth or rubber covered bit or it was war, she hated metal touching her teeth at all.

I feel like I might have wandered off on a tanget, I'd try lots of positive reinforcement when he doesn't overreact, and slowly ease him into situations where he might, otherwise, overreact and teach him its okay by tons of treats and repetitions.  Hold the halter tight, bring his head low, and treat.  Rinse repeat.  Maybe start in a situatuation where he feels it's a little more easy to escape, like a roundpen.  You don't want to make him feel trapt, you want him to feel comfortable and accept it as a new normal.