If you know that there are no health issues, then it's a behavorial one, maybe left over from some time when the bit was painful. in any case, let the horse discover how bad it is to do this. you let him bop himself in the mouth. You have your hands fixed, maybe against his neck, and your core engaged so that he cannot pull you off your balance. give him enough rein so that he can stand quitely with no rein contact at all (afterall, he's just stnading, right?) If he tries that manuever to get the reins out of your hand, you maintain your position very firmly. He will bop himself in the mouth, and when he puts his head back down, he gets his own relief. you do nothing except give him NO more rein than enough to stand comfortably with his head up , in a natural position. He will only hurt himself by doing this, but you have to be very firm so that he never gets any reward for this type of behavior.