I would suggest to NOT show the back pocket to the batter ever. That is over rotating and will create more issues to correct. You do not fix a problem by over compensating and creating another.
I agree that you should not throw the "kitchen sink" at your son all at once. Pick 2 things to focus on and correct those before going forward. The top 2 IMO are:
Hips: this is and can be an easy fix actually. Film him from the side (back) and make sure he is getting all of his weight over his back leg. During leg kick, make sure lead foot does not cross back leg BUT here is the fix.......the lead knee can go back to SS (point that direction. What this does is allow proper positioning for weight transfer to the plate and proper positioning for the lead HIP. By pointing slightly to the SS, it allows the hip to naturally lift and then at the beginning of weight transfer, the HIP is what leads first and this keep the lead shoulder closed too. So both the hip and the shoulder are falling toward the plate closed. the lead foot should then land towards center line (slightly to the left) with the lead foots toe slightly to 3B. Only upon firm planting of the lead foot will the hips then get involved in rotation.
Glove forearm should be parallel to the ground, should height with the elbow/armpit 90* from armpit/hip. As hips rotate after plant, pull the glove to the chest to help in overall rotation. This is what they refer to as the "complete rubber band" effect.
The head should be over the landing knee or atleast within one head's width of either side of the knee. This will stop the falling off to one side and help with control and less risk of arm drag which isolates both the acceleration and deceleration areas and increases potential for injury. The reason why you have arm drag now is because when the hip/shoulder flies open, this makes your kid throw "open" ( to the left of center line) and this in return is a domino effect to arm placement which makes him drop slightly and "slingshot" around since he does not have the core and lower body working properly for him.
Correcting the mechanical defaults will indeed add velocity without any more force on the body. It is standard to expect 5-6 mph up to even 10 mph if you correct these issues. Just make sure he doesnt buckle (drop/bend) the back knee when correcting.
This is not a fix in 20 reps. It takes countless throws from the mound. I suggest a dirt mound that you rake every 10 throws so you can see where he is landing from center line.
Good luck
For Love of the Game