You got some really great responses here. :)
Lacey was pretty "broken" (not in a "dead broke" sense but in a "mentally gone" sense) when I got her. Her previous owners were very very back and forth with her. One moment she'd have zero rules and the next minute she was being "idiot-ed" (LOL!) for breaking a obscure rule that hadn't existed previously.
One person, the woman, her "real" owner, would let her do anything. There were no rules, it was just "what does Lacey want to do today?" all the time.
That woman's sons were the exact opposite. They expected perfection, without training, from the get go. They also wanted to do a ton of running, which is the worst idea ever with a hot young Arab.
Anyway, when I got her, she had spent 23 years with these people. Thankfully they had retired her when she was 10-15ish because she was "out of hand" so she was able to forget some of what they "taught" her.
She had learned that mistakes were the worst and should never happen, and she should never try new behaviors because they would end in mistakes and she'd probably die. If she happened to make a mistake, she could just turn into a whirling dervish and scare her rider into getting off, therefore escaping punishment! I'd just try to cue her for something with no goal in mind, she'd do it imperfectly and FREAK OUT. She thought it was a pretty handy plan. And it had worked for her!
Anyway, I learned that I needed to be as calm as she was excited. I once read someone on here describe it like this: if your energy and your horse's energy were numbers, together your total number should be a 10. So, if your horse is being really crazy and excited, perhaps an 8, you should be just as calm and laidback, a 2. If your horse is being super lazy, a 3, you should be energetic and getting him/her involved, a 7. And so on.
At first, I'd just get on and sit there (since Lacey's favorite habit back then was to freak out, via bolting and rearing as violently as she could, as soon as anyone got in the saddle), with absolutely no energy. It took a while for me to master (try 8 months! lol) but it's almost like a sort of medative state. Like, you're present on the horse's back but you aren't really influencing the horse beyond keeping him/her from hurting themselves. You're just sitting there, not really requesting anything, just letting the horse figure out that they aren't dying and that you're safe.
At first, with Lacey, she'd get done with her original freaking out but as soon as I requested that she do anything, "Captain Freak Out" was back. For a few months, all we did was I'd sit there, on her back, and she'd go from one thing to the next, freaking out. Pretty quickly, her episodes became more and more brief but she'd still have little fits over everything.
After a few months, she finally figured out that that scaring me thing wasn't working too well so she started just doing stuff when I asked her to do it.
She's still triggered by certain things and she still isn't comfortable with mistakes but she's learned that I'm not going to push her too far or annihilate her for doing something wrong so she will give a "best effort" try at least once to most new things I ask of her. If she fails that first try, it's very difficult to get her to try again but she's coming along. Now I can push her buttons waaay past what I used to be able to, it just took a lot of time and trust. There are deep down buttons I could push, and don't, because I don't feel like dying but thankfully all her easily found buttons have been desensitized.
Anyway, just remember to keep your energy low when his is high and you'll be good, Pretend you're one of those dummies people sometimes strap to young horses to get them used to weight on their backs before their first rider gets on. Or a Buddhist Monk, horseback riding style...
Good luck with him! :)