Where you are starting (At the beginning, with TONS of groundwork) is the best place I can think of...a horse like that has some seriously sticky feet...it really doesn't matter "where" the issue stems from (disrespect, fear, pain, you name it...when a horse rears, it's him telling you, he basically feels he has nowhere TO go, but up in the air).
I would also double check to ensure that this isn't a pain related issue; back, teeth, hips, whatever...just get a through vet check to make sure that the issue you are experiencing isn't him telling you something actually hurts. Yes, rearing is dangerous, but there are times where it can be pointing to something other than a training issue.
Other than that (groundwork and vet check), I really don't have anything else to offer; I have never had a horse (rearer, or bucker) that the issues didn't resolve when the groundwork reestablished proper working respect and trust between horse and handler. The biggest thing here is teaching him that while he is in your control you control his feet, and somedays you will have to put some hustle in his hooves to make that point; it will mean some sweaty pony days, as well, but if you do the work properly (not just working him to tire him out), you will have a much more well tuned, well mannered horse in the end.
Some folks will use an egg or water balloon on the horse's head as he goes up, but really when a horse is going up in the air, that is the last thing I am thinking (to try and whack him between the ears with something! I will be making sure my body remains balanced and my hands aren't going to yank on his mouth and pull him over!!!)
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."