The surface condition that your son performs off of can influence his pitch(s). Take for example this mound below...
More often than not, he'll be standing and looking down at these conditions:
And these conditions can influence his form and posture.. before/during/after his delivery.
If your son's pivot foot is in a hole.. his toe will be pointed downward and at that place in time he's already behind in the count .. before he's even started his pitch. Try digging a hole in your back yard and placing YOUR pivot foot in it. Noitice your toe pointing downward.. thus shifting your weight to one side as you try to throw.
Also, even a well groomed mound or pitching surface requires good stride leg discipline(sd). Too many youngsters and head coaches in the youth game take this stride leg for granted and not giving it the credit that it deserves for influencing control issues... up-down.. left-right accuracy.
The picture below will give you some guide on the stride leg's contribution to control issues.
And last but not least... what TARGET is your son concentrating on? He should NOT be pitching to an abstract of the catcher/umpire/batter. He should focus on the mitt, the catcher's mask, top of the catcher's knee, etc. and all that depends on the pitch at hand and his control issues on any given day. FOCUS ON A TARGET ... some pitchers find it helpful to start with the catcher's mask and then work from there.
Best wishes with your son's progress..