Is it his weight distribution? Also seems to rush to the plate a little bit. Misses high
I think his overall mechanics and arm path is solid. The only thing I see that could cause him to miss high is slowing up his back leg at the end, instead of using that power to work down the mound some more on the follow through.
One of the first things I look for in a pitcher is weight shift/ early momentum. I look for early weight shift while getting gathered and loaded and leading with the hips/core. I also look for momentum towards home plate before hand break. Although not a must but a good rule of thumb. Along with this first move, I look for the pitcher to be stable and engaged with the back leg with the center of gravity and hips to lead the action.
I see 2 flaws in the beginning of his delivery. One of the flaws is not your son’s doing but the mound he is pitching off of. Look at the bottom of his foot. A pitchers entire bottom foot should be flat on the ground as long as possible so he can use ground force to push down and create good leg drive. Because the mound forces your son’s foot on an angle, it pushes his knee in which causes early rotation. This makes it hard for him to create proper momentum towards the catcher.
The second flaw I see is his lead leg. He turns his knee out and leads with his knee instead of his hip. This may happen because of the mound. Place him on flat ground and video him. Compare the 2 and see if he still leads with his knee. If he does, have him perform drills where he is conscious of the bottom of his heel driving to the catcher.
I included some picture so you can visually see the difference