Agree with Coach Paul that he needs to incorporate his body more and create momentum. There is very little being created with the lower half. I would definitely prescribe a lot of the Hershiser Drill. Good call on the head tilt. That will take a lot of repetition and added strength to correct.
The stance he uses in the stretch position (3rd video) is concerning. His feet are way too far apart causing him to be off balance and get nothing with his lower half. He is relying on all arm. The feet should be brought inside the shoulders. It almost looks like he is trying to use a slide step because there is no knee lift. I don’t like youth pitchers using a slide step. I would rather have them pitching with proper use of their entire body and develop strong pick off moves to control the running game (especially for a lefty). Attempting a pick off move with his wide stance would be really difficult.
Good luck. Sounds like you are doing your homework!