Try to determine when and why he behaves this way. Check the equipment to make sure everything is the way it should be.You may have to retrain your horse. Take the reins in your left hand, holding firmly but not tight.Put your foot in the stirrup with a light hold on the pommel. If your horse moves, gently take up the slack on the reins, then start all over. Reward him when he behaves. Don't mount until he stands still. You may have to repeat the sequence a number of times until he gets the idea. Once you mount, wait to move. That will help him know he must follow your lead.