from the stretch, his upper half comes through all at the same time. He does stay closed longer from the stretch than from the wind up, so his stretch looks like it would be more efficient than his windup. Regardless of position, his separation of hips and shoulders and his external rotation are points to work on. His hand break is a bit matter-of-fact when hand break should be where his intent to throw hard should originate. As his hips rotate, the ball and his glove should really not change position. Only after the elbow comes forward to the chest should the ball begin to move forward. Imagine a waiter carrying a tray of drinks over his head. He's got little to no forearm layback or shoulder external rotation. Major velocity choke point for him since his hip energy is being blocked by his shoulder action.
He can get a feel for how it's supposed to be by using a slightly weighted ball (certainly less than 2 pounds would give the feel) standing in landing position and holding the ball in the high cocked position then work on keeping the ball from gaining ground to the plate during shoulder turn and external rotation. As his shoulders and elbow turn and square up to the target, his forearm will lay back to allow for the ball to stay relatively still. This is not a throwing drill. It's more of a plyometric exercise that can be done before and after throwing but not before your warm-up. I use a 6 pound ball, but I'm much bigger than you are and my rotator cuff is solid.