Quick fix? Go to the slide step. No leg kick. Have him turn his strike toes towards home at the very last second. That will fix it for now. The video shows him bring his leg up and at the top, he turns his knee and foot towards home (at the top). This allows his hips and shoulders to open way too soon…thus creating wildness and limited velocity, I assume. He needs to keep his lead (strike) knee and foot closed and bring his foot down to almost the ground and then move forward.