If this horse is trotting off really fast, and refusing to stop just because you pointed the whip at his rear end, he is not desensitized to it, not completely anyway. Especially if he's ignoring it on his shoulder, but scared of it on his rear. You need to start over and desensitize him to your whip, remembering to keep your body language in a very neutral tone. Stand up straight, and avoid direct eye contact with him while you're doing it. Remember to keep his head tilted toward you while you are doing this. You should be able to rub him everywhere with the whip and sling the lash all over him, including around his legs. Just remember to use very light pressure in this phase of training. Once he is truly desensitized to the whip, lounging him should be relatively simple, change your body language to a more aggressive, direct eye contact, and shoulders slightly forward. With the hand holding the lead, point in the direction that you want the horse to go, then tap the air with the whip, if he does not move off, tap his rear lightly, keep progressing with increased pressure until he moves any. Once he moves change your body language to neutral and rub him with the whip again, then start over. He will eventually move around you like he is supposed to, with just a point of the finger. It takes a little time and consistency, but it will work.