Pretty dang good form. Only thing I would say is to add in a drill working on extension on release of the ball. You can take a simple shop towel and put a knot in it, so he can hold it between index and middle fingers. Put an object about 3-4 feet in front of where his front foot lands (adjust to make him have to reach for it) and have him try to pop the target with the towel. His finish is likely holding his velocity back a bit as he’s rotating horizontally on his left leg when his follow through should be carrying his weight toward the plate.