Every time you get on him and he rears, wack him as hard as you can with a crop between his ears
This works, when it works, only if you time the "whack" when the horse is coming up off the ground. The reason that it works, when it works, is the horse believes it has hit itself on something overhead that it can't see, and become reluctant to lift it's front end off the ground for fear of hitting itself again.
It's not a technique I would ever recommend to less than a very experienced, confident rider because it's very hard to do correctly and very easy to do wrong. Done wrong, all you're doing is reinforcing the bad behavior and increasing the liklihood that the animal will flip over.
I also would never recommend this technique without thoroughly investigating the root causes of the behavior. It's highly likely that this otherwise sweet and agreeable pony has a reason for this behavior and is trying to communicate it to his rider.