Tagging onto Cabbagedad’s comments I would add this.
Almost all pitchers struggle to stay closed as they travel down the mound and your son is no exception. The chain reaction of flying open too soon always begins with the toe of the stride foot. When a pitcher flips the toe open and points it toward the target it causes the lead hip to open too soon, and then the front shoulder follows by also opening too soon. When you combine the front side opening too soon with the extremely long arm action that your son has on the back side the arm can’t catch up to the position that the body is in and the common miss is high on the arm side. So all that is what’s going on.
My advice would be to work only out of the stretch while trying to implement change. Besides starting in line and staying in line there are other key things to focus on. He is very loose with his glove side mechanics. That needs to be tightened up. The stride foot should land slightly closed (think 2 on a clock). The arm action in the back should be shortened up for a more efficient and quicker delivery - IF POSSIBLE. Some pitchers just can’t adopt a shorter arm action - but they are better off if they can. When stride foot strikes the hips should open before the shoulders. This hip/shoulder separation creates torque that increases arm speed. Your son appears to have the flexibility to do this well. The back hip needs to get up and over the stride leg and not spin around it to the 1B side. The chin and the chest should extend toward the target as long as possible as should the arm after release.
Besides all of that, here is the most important piece of advice I can give you. Find a qualified pitching instructor to teach your son the things that have been discussed in this thread. Don’t try to do this yourself. Most likely your relationship with your son will suffer if you do. Ask around and find the best person in your area. Don’t take the first recommendation that you get. Find the best HS & college pitchers in your area and find out who they work with. Get references and check them out. Do your homework.
Best of luck !!
