I'm glad I was able to give you hope! I truly understand how hard it is to bring a "misused" horse back from the brink (or in Lacey's case, euthanasia)
Lacey actually did a similar thing when I first got her and I tried riding her. She would buck and rear and basically do everything to try and scare me. She never bucked hard enough or reared high enough to get me off, that wasn't her intention, she was just saying "HEY!!! I'm really scared of what you're going to ask me to do! Please don't scare me more!" I actually had to give up on riding her for a few months and I just worked with her on the ground. We did lots of lunging and leading exercises and basically just got to know each other. Once she figured out that I'm benevolent on the ground she realized that I'm benevolent on her back too. When she'd act up I'd just sit on her, not asking her to do anything except walk calmly. I wouldn't engage the reins, I wouldn't touch her sides, I'd just sit there (grabbing mane most of the time because I felt insecure, lol) and she figured out that nothing she did could cause me to punish her or get off. Now I can hop on bareback in a halter and she's good to go, she likes to have "discussions" still but she knows that I wouldn't ask her to do something she couldn't handle (which as it turns out, is what she's the most afraid of).
I found out this last summer that her previous owners "trained" her using a sort of ask then strongly demand (demand using way more force than is ever necessary) strategy, even though they knew she had never been taught the things they were asking for. Some horses just shut down with that sort of thing but Lacey, and your boy it seems, learned to fight. She still fights sometimes, that has been in no way removed from her personality, but she fights more softly. She picks her battles instead of turning everything into a big fiasco. More importantly, I've learned how to deal with it. I keep breathing, keep my energy low, I sigh a lot (I've found that Lacey loves it when I sigh, she'll generally sigh right after or soon after I do, releasing the built up tension), and I just calmly ask her to do things I know she can do, like going in a circle. If I start fighting her back, kicking her or smacking her neck, that just brings me down to her level and makes the whole situation worse because "OMG! Mom is getting mad! There must really be something dangerous going on here!"
He'll come around, no worries. =)
What kind of bit are you using on him? If he's not in a single or double jointed snaffle already, I'd suggest putting him in one, atleast for the time being. Good for you for checking his saddle fit! That was a big issue for Lacey too. She is hard to fit with high wide withers and a wide shortish back, not exactly your usual combination. I had been riding her english but I switched to western for the summer and guess what? The english saddles don't even remotely fit her anymore, they were actually hindering her muscle growth before and I just thought that they didn't fit perfectly.
