From the wind-up:
Posture: as long as he’s pitching from the wind-up, he should be standing facing straight toward the plate with both feet at almost shoulder width apart. He’s turning his body away from the plate- not good. When he turns his body away from the plate, it loses momentum toward the plate and he must use extra energy to bring it forward. Keep his body straight, and do not turn away from the plate.
Make sure his head is stable, keep it over his center of gravity (belly-button).
Leg lift: he’s definitely bringing his leg too far behind his opposite leg’s knee. When he comes to the point where he lifts his leg, he should just lift it straight up, not up and backwards over his other knee. (when he brings his lifted leg backward, it transfers his energy behind him- you do not want that, you want energy and momentum only toward the plate).
Stride: I’d say he could lengthen his stride another foot. A longer stride will help him produce more momentum and energy toward the plate, thus boosting velocity. Now, I don’t know how tall he is, but it would be a good idea to measure him and eventually work to get him to stride a full 100% of his height. It will help his velocity as long as he can still control his body.
Arm action: I would say that he is separating his glove arm from his pitching arm too early, much too early. This accounts for the pitching arm going low to the ground and then coming almost over the top. He should not be separating his glove arm from his pitching arm until he shifts his weight forward following leg lift. What he is doing now with his arms is putting extra stress on his elbow and shoulder- change nothing and he faces future arm injury for sure. What he needs to do is delay separating his arms until weight shift forward- then keep his arms “equal and opposite”. His pitching elbow should not come above his shoulder (if it is above the shoulder, it puts more stress on the elbow and shoulder). If his elbow is below his shoulder, he will lose velocity. If it is above his shoulder, it will cause eventual injury. So just keep it practically level with the shoulder, it’s much easier on the arm.
Enough for now- hope it’s not too confusing.
Post your questions if you need me to clarify anything.