The most important thing to do when you know you have a rearer and an impending rear, is just to get the feet moving, doesn't matter where. Getting them bend laterally is a good start, because if you can get him to turn, you are going to offset his balance, so he can't go up. Teaching him to turn on the forehand, can help too, as that keeps his weight ON his front legs...which equals no rear.
But, I do not agree that rearing horse cannot be cured; no it can not be done by whapping him on the head...that is just a punishment, not a cure. But by teaching him to engage his body, and thus his mind, you CAN gradually get him out of that mindset that says "I'm scared, or confused, or whatever...I have to go up"...rearing is usually a lack of proper training, not just a bad behavior. It becomes a bad behavior when an owner doesn't know how to retrain the horse NOT to do it. And training a horse not to rear has nothing to do with actually addressing the rear...it has to do with riding 'smart' and knowing when the horse is in a situation he may go up, and being one step ahead of the horse, in order to avoid it, by putting the horse's mind to work.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."