First thing to look at is what he does prior to the head movement and, in particular, his set-up on the mound. Make sure he’s not doing something earlier in the delivery that leads to the head movement.
Also, look at his tempo. The difference between pitching and playing in the field is that fielders usually need to get rid of the ball as quickly as possible so they move quickly through their delivery while pitchers can move a slowly as they want. You might want to have your son get himself moving down the hill sooner/faster.