I don't know how long you've had him, but I would build a relationship with him so that he knows you, trusts you and sees you as leader. That can reduce or eliminate a lot of overreaction and will help you increase the odds of bringing the horse back to sanity in a loony moment. Once you've established that, then I would start exposing him to things. Quietly at first, then escalating. For example, I wouldn't charge at him with a plastic bag, but I would hold one and let him sniff it, let it hit his legs, let it tumble to the ground, etc.