I’m not a Mechanics coach but I do know one thing that your son should do.
Make sure his pivot/back foot is on the ground during his release.
I don’t know much else but there’s one thing.
Needs to lead with hips not front foot.
First off, I love how he takes a deep breath before he pitches. I try and tell all my pitches to take a deep breath and focus before each pitch. As with all young pitchers including Chuck, they have to learn how to use their bodies (lower half) to help gain momentum into the stride. That was the first thing I noticed, lack of early momentum. Most young just pitchers just like Chuck take the ball out of their glove, go through their delivery, and throw. What is missing is the lower half mechanics.
I know he is only 9, but developing good movements now will only help him as he gets older. I would like to thank Phil Rosengren for the information I learned below. Start with some light catch to prep the arm for throwing… let him work on weight transfer a bit, and get a feel for the ball leaving his hand the right way (staying behind the ball). Help him understand the pitching delivery as a whole, then break it down into parts. Gradually work to put it all together. Don’t make it overly complicated.
You don’t need to work on all of these things all at once. An example would be to work on Glove-Arm and Throwing-Arm Connection. Next time out reinforce what you talked about, then maybe spend time working on getting over and around the front leg. Then when he get that discuss and show him the load and early weight shift.
After you’ve worked together to help him build good movements in his delivery and he fully understands it, the next step would be to put it all together. So how do you do that? Start with dry work without a ball. Anywhere from 10 to 20 reps is usually a good place to start to help him get the feel for repeating his delivery. Emphasize focus and awareness. Then I would progress to flat ground pitching. Once you get to the mound, repeat the process, dry work to get used to the mound.
Most importantly, keep it fun for him.