your son needs to have a better "feel" of where his hands are as they enter the launching area. to assist in that regard have him try this; as he sets up, touch his chin or somewhere in the neck with his top hand thumb. That will give him immediate feedback and awareness that his hands are both at the top of the strike zone and positioned near his rear shoulder in a better pre-launch kind of inside "floating" area just prior to entering the launching slot. it will also assist with bat lag and to keep his hands "inside the ball" to release point.