I see quite a few good things going on in his delivery as he starts lifting his pitching arm up, but it is hard to really see what is going on in slo motion. If you have real speed load that up so we can see. What I do like is:
- His fingers are on top of the ball
- Back foot is planted on the ground using good ground force
- Staying closed, and good glove arm action.
- Keeps his center of gravity over the driveline to home plate
- Hips open into foot plant
- Front leg stabilizes
- Hips get all of the way through
What I would suggest working on is early momentum. This is what I consider the first power move in the delivery. I look for early weight shift while getting gathered and loaded. Leading with the hips, Momentum towards home plate before hand break. Stable and engaged with his back leg, and the center of gravity and hips lead the action
Your son is doing a slide step so I want to put up an image of a pro player in the same arm position doing a slide step utilizing ground force and creating forward momentum.
Hope this helps