Very nice looking young man and I really like his engagement with the game. He is a good candidate to go ahead and teach to cover or back up on any contact because he has that level of engagement now. He shouldn’t just be watching.
Coach Paul gave you a lot of good stuff.
This is how I would approach implementing his advice.
Only pitch from the stretch.
Both feet firmly on the ground when getting signal or acquiring target if no signals are used. He should have his posting leg knee flexed inward enough (but firm) to feel the body weight on the inside arch of his posting foot. His feet should be a little wider; closer to shoulder width.
His lifted knee should come up (for now just about waist high) and towards the center line of his body. This will close his hips up slightly. His front knee can then start to lower with the side of foot towards the plate or even slightly heel forward. His back hip should then drive his front hip directly toward the mitt. The Hershiser drill will give him a good sense of how this should feel, but you don’t want him to just pop the hip forward. It should be driven forward by the back hip and not from the back knee.
His hands should start to separate just after the lifted knee begins to lower for the stride out.
I like his arm circle up and he is fairly opposite and equal. I prefer he break his hand with the back of his throwing hand toward third.(non teach per some coaches) He currently breaks palm towards third. I would also like to see his glove thumb down as his breaks his hands and then turn over as he extends his chest to throw (or scap loads), as I think it would help his front should stay aligned longer.
Once these changes were made I would let him be for a bit and just have him concentrate on balance and intent without over throwing. By balance I mean dynamic balance where he is able to move smoothly and quickly directly towards the mitt.
I would also use these same principles for developing a little long toss routine for him so that he is throwing with effort. Trying to throw the ball long and loose.
I think this would put him in a pretty good spot to build on.