mane if you need to. Give the horse all the lead line it needs to rear up.
By keeping your attention on the horse and the horse's attention on you at all times, you should be able to not have to deal with rearing in the first place. It can happen, however. So if your horse does startle or pull back or rear, don't punish the horse. When it happens, you simply interrupt them with instructions of what to do next.
A horse should be having a good time. If he gets startled or frightened, you want him to come to you as the safe place to be. You want to be a person he can trust.