I think using a whip or a dressage whip will work wonders, but you don't want to make him afraid of whips by wacking him with it! You could try just annoying taps at first (while facing forward and putting a little forward pressure on his halter) start light and if he doesn't respond continue to tap a little harder each time until he moves. I totally understand that wacking his would be similar to what wild horses do to each other but I know that my horse can get really skiddish and if I wacked her with the whip she would just be terrified of it, something I wouldn't want. Of course, all horses are different and maybe your horse wouldn't care? Just a suggestion. Good Luck!!