So yesterday my instructor tried to trot him around the arena, against the fence, with the dressage whip, and even SHE had trouble. She finally did get him to go a few steps at a trot (after a little temper tantrum). After that she got him to trot once more (with a lot of effort), but he pretty much took off at a high speed trot and flew out in front of her.
I feel a little better that it's not just me. Hopefully now he's at least gotten the start of an idea what we're asking.