This is my humble beginner opinion.Your stride foot is landing to open. Your glove side arm is moving too far from your body.Coachs feel free to critique my critique.
Yes - landing too open and not leading well with the glove arm… in addition:
Not getting enough mass behind his pitches - lead more with the hip and keep the weight centered more over the back leg.
Break the hands lower, later and closer to the trunk (belly button) - his arm is getting into throwing position much too early.
Work on getting full back leg extension - there is a lot of energy left in the tank there; don’t turn the front foot until the back leg is just about fully extended - then put it down.
Get the glove arm up before the throwing arm - use the glove side upper arm as a “site” like looking down the barrel of a rifle at your target rather than flailing it out to the side.
Structuredoc, you have hit on the exact thing I’ve been talking about all this time—the pitcher has to get his whole body into the action, using the legs, the hips and the torso in one continuous motion. Doing this enables the pitcher to generate more power behind his pitches, and in the process take a lot of pressure off the arm and the shoulder so he can throw harder with less effort. There are a number of drills and exercises on both this website and the NPA website, and one of the best is the “Hershiser” drill which aims at getting the hips fully involved. If this kid works on some of these things, everything else will fall into place. 8)