Congrats to you as a parent for your son enjoying the great game. Although I was a professional infielder and hitter, I have coached a very select 13 yr old team for a number of years.
All I can see from this short video is that his arm is “dragging” somewhat at the time of his downward delivery. i would suggest, if you haven’t already, to get him lessons with a respectable pitching instructor that actually experienced success in the game. He seems to be tall and lanky for 12, which is a good thing. My other advice to you is to make sure he is having fun and enjoying the game. This is absolutely most important, in my opinion.
My best to you and your son.
Alex at coachposting.com