Stand outside the stall where you are safe if he kicks. Reach in the door/gate with a lunge whip and tap his butt. Be real annoying with it and do it in a rhythmic fashion. If after about 30 seconds he doesn't move his hip away then tap a little harder. Be patient, he'll move. As soon as he even rocks his body, stop and point the whip down. Give him oh, 15 seconds and if no further response, start again. He's trying to figure out how this works.