Hey, termhn, it sounds as if you've been picking up on what I've been calling "The Secret"! Good for you. After reading your take on what the kid has been doing, I am convinced that this is what he needs to do. In a nutshell, he needs to get his whole body involved in the process of pitching so that he isn't throwing with just his arm. I learned "The Secret" many, many moons ago from watching how the Yankees' Big Three pitching rotation did it; they were all driving off the lower half of the body, using the legs, the hips (oh, yes, the hips) and the torso in one continuous motion, creating a nonstop flow of energy all the way up through the shoulder and arm, generating more power in their pitches (even Eddie Lopat who was not a fireballer), and not only throwing harder and faster with less effort but also taking a lot of pressure off the extremities involved.
A good place to start would be the "Hershiser" drill which aims at getting the hips fully involved in the process. It requires no special equipment, just a fence or a wall, and working with this will speed up every aspect of the windup and delivery. Gosh, I just love talking about this, even after all these decades.