Okay, this brings up another question. Do you know how this horse was trained for WP? If he was trained to have a "spur-stop" then, to him, more leg actually tells him to slow down.
If that's the case, then the re-training is basically the same as I posted earlier, but I wouldn't get after him quite so harshly at first. Learning a whole new set of cues that is so contradictory to what he's been taught before is terribly confusing.
If the spur stop is his problem, he would likely respond to a brush or a tap with the whip instead of an all-out wallop.