Long as they are having fun! 9u hard to get too technical on them, they are learning. If you want very simple things to work on that will have long term benefit that I noticed. He doesn't look steady on his plant leg, you mentioned he had some sidearm you were working on which could be some of it. His weight seems to go off to the right, and he is pretty straight up and down, not much follow through. Below is a video of my 9u boy from first game this year, his pitching coach works a lot one simple things he can focus on repeating. Weight going toward target, balanced on plant leg and balance point, then when he finishes, follow through to where he could touch the dirt with the fingers he was griping the ball with, aka to where coach can stack plates on his back. Easy things to work on, not too complicated, I'll get some video of his next bull pen too. I like it, good stuff without being overly complicated for this age.