You're going to have to break down the motion into a couple of "pieces" and have him do it over and over again. Example, first he does his stride and stop at the "equal opposite" portion of the whole delivery. Glove should be pointing at the target with the ball pointing straight back towards second base or 3rd. He'll have to do it over and over again. Then move onto the next portion of the delivery, one step at a time. It's going to be hard changing this, because he's going to have to "think" about pitching again. Right now he just throws the way he always has, he doesn't think about any piece of his delivery. It's all muscle memory. The only way you'll really break this habit is to make something else muscle memory. He's going to pitch "bad" for a while, most likely. It'll take all season unless he's an advanced motor skills learner. This is just my opinion, but that's how I'd go about doing it.