First post here, but I wanted to give my opinion.
I completely agree with the other arm action comments on the first video. He seems to lift the ball up to the high cocked position, and then it pauses for a second which creates a loss of momentum (this pause isn’t as evident on the second video). Keep working on making the arm action one continuous motion. Thinking about breaking the hands later may help naturally force this action to be smoother and quicker (quicker in terms of the speed of the arm, not the timing aspects of the arm action). Attempt to separate the hands by separating the elbows in a quick smooth motion instead of swinging the ball in a circular motion. After the season is over, you may want to work with him to be a little quicker with his delivery so his hand/arm is forced to speed up and doesn’t allow time for the pause.
The other thing I noticed is that he appears to go linear at release. This is more apparent in the original video. The rotation of the shoulders stops a little early and isn’t completing the throw. It looks like he still has momentum carrying him forward that should have gone into rotation around the front leg. The second video looks a little better in that regard, but something still doesn’t look right to me about the way his throwing arm finishes. It almost looks like his arm gets out of the rotational plane of the shoulders too quickly. Again this tells me he is going linear and pushing the ball instead of whipping it. It almost looks like his arm finishes in a manner that is similar to the way many pitchers look when they are doing the old towel drill for extension.
Don’t get me wrong, he does a lot of things well for his age and has improved since that first video. These are just a few things that I would work on.
Hopefully this information helps somewhat. I’m a little rusty because I haven’t worked with many pitchers in the past few years.