I used this video of your son to get a better look at his side view:
I do notice at :03 seconds of this video he defiately has a collapsing of the hip and a odd rotation which causes his foot and knee to turn in a strange way. I think that this action is holding back his ability to totally turn his hips and trunk quickly and explosively in order to get the most out of his velocity.
I would try some things that might make the issue go away simply. Has he ever kept his head on the target at release or has he always pulled his head down and to the right, this simple action of keeping his eyes on the target might just do the trick.
Next, the glove side could pull at least back to the chest or even more by pulling the elbow back in a kind of chicken wing (isn’t the most popular but it might help pull the ball through and toward the target).
If these don’t improve the action of the legs, then the next thing would be to pull his foot straight off the rubber…this would take more effort and time so I would try the other simpler fixes first.