Could be a lot going on there. Is he just missing, or is he pretty wild? This could be a small adjustment or a mechanical flaw that results in a motion that is difficult to repeat?
A little different scenario, but it may work for you. My son had the C yips pretty bad and couldn't throw the ball back to the pitcher. We started playing light catch and slowly advancing to full speed throws with the lightest Driveline baseball. It is a 3oz ball and forces the player to make adjustments just to get the ball to the target. Then muscle memory kicks in and the mechanics become repeatable.
Whatever the issue is, I'd recommend a different pitching coach. If he hasn't been able to diagnose or get your son where he wants to be in year, I don't see him getting him there at all. After a year they should be focusing on small tweaks to get him better and developing/enhancing other pitches.