For a complete stride alignment to home plate: first have him throw a pitch and mark the spot where he plants his stride foot at release. Then have him get into the stretch position and place his front foot where he planted it at release; with his hands separated about shoulder-width apart. Next, have him take an extra step and throw from there. His legs will begin to get a natural feel for striding out beyond his normal planting spot. From there work him backwards by having him come set after he has thrown from that point a few times. Sometimes working in reverse with mechanics can give you a better understanding and feel going forward with mechanics.