Get a spray bottle of water, a long lead and halter, take him to a safe enclosed area and start desensitizing him to it. Start with a spot he feels okay about and work from there. If he rears or moves away, keep spraying until he stops and relaxes. THEN you reward him. If you're consistent, he'll learn that rearing isn't what makes the bottle go away.
For a quick solution, spray a rag and wipe him down instead of spraying that area.