Here’s a couple of observations you might consider. In the second video - if you stop it at the :29 mark you can see he has planted his stride foot and is getting ready to launch the ball. The first thing I see is that his stride foot has fully landed, yet his hips have not yet begun to turn - however his glove side is flying to the 3rd base side which will cause his shoulders to open pretty quickly. Not a good combination. another thing I see is that his stride foot is extremely closed off which will also impair his ability to open his hips earlier. Some guys can get away with this if they compensate by having an aggressive backside hip. Your son does not as you can see his front foot does not turn toward the direction of the catcher until after the ball has been released and into follow-though around the :32 mark. Basically, what you are seeing is his shoulders pulling his hips through instead of the other way around.
I’d start working on the lower half with Hershiser drills and rocker drills. A quick search on this site or google should get you video of those. At the same time, I’d have him work on controlling his glove side. Look up “NPA swivel & stabilize” for a good understanding of what needs to happen after separation.