I have never been a fan of players both pitching and catching. My first advice would be to pick one or the other. If he/you/coach insists on do both here are some suggestions on the pitching side. Work exclusively out of the stretch until he is commanding the baseball. Make sure that the length of his stride is at least 90% of his body height. At release have him focus on letting his chin and his chest keep moving toward the target as long as possible. This will help him get better extension. Tell him to try and finish with his back parallel to the ground, head up, eyes on the target. My suspicion is that he is too upright at release and is cutting off his extension toward the target. If necessary, tell him to exaggerate to the point of throwing the ball in the dirt to get the feel for a different release point.