This horse needs a lot of groundwork to sharpen up his manners. You need to keep his feet moving, turning him away from you, not toward you. We are all guilty of turning the horse toward us but that helps him assert his dominance. He thinks he's moving you. Make all your turns by driving his front end away from you. Keep a dressage whip in hand at all times. If he rears, deliver the hardest hit you can, be it his shoulder, his leg, his body. So hard you think the whip will break. He needs a strong reaction for his action. He may learn from the one time but he may need it again. He will know that rearing results in a hard whack. If he's a slow learner he might do it a third time, but I doubt it. But know that just once may not fix it. Walk him in large circles so that you keep pushing him into the turn. Hold your whip toward his eye/jaw to move his head away. He will eventually start reading your shoulders and where they are going. Do some backing up. As long as his feet are busy it's hard for him to plant his hindfeet, rock back and rear. Get real good at moving his hips laterally. As long as one hind leg is crossing underneath he's like a 3 legged table, a little tippy.