Very good mechanics especially for his age.... when he is throwing from the stretch his legs aren't being used to their full potential ... have him separate his hands AFTER he feels his hips shift forward a few inches toward the catcher as he is coming out of his knee lift ... tell him to separate his hands after his hips move forward which creates better momentum and acceleration as he strides out. Use the Yankee Stadium Drill to reinforce this technique: # In the stretch position stand with the throw-side shoulder against the wall. # Then go into a knee-lift and slowly shift away from the wall at the same time. # separate your hands after your shift starts while going down and out and if his throwing hand hits the wall then he is separating too early, if he is able to bring his throwing hand up-and-out to the throwing position without striking the wall then he is separating behind the powerful stride of his legs allowing for less stress on the arm and added instant velocity. If you are outdoors then stand behind him on the mound to imitate the wall.