He lifts his knee and brings it back down to the ground before he gets his lower half moving towards the target. As a result he is limited in the forward momentum he generates. If he gets his lower half (butt moving to the target) sooner,faster and more aggressive I think you would see a nice velocity increase. He looks to have good size and athleticism for his age, but he isn’t using enough of it. The Hershiser Drill would be my first recommendation. Good luck!