Something you might consider are his feet as he takes his stance. Many times hitters open up their front foot after they stride. This causes the hips to open up prematurely, and then the chest, front shoulder, and head all pull off the ball in sequence. Try having him point his toes inward toward the plate in his stance, and then be sure the front foot is still pointed inward after the stride.
You might pair this with outside pitch teework. Another drill that may help is the alley toss drill. It works like soft-toss, but the tosser is directly behind the plate, maybe 8-10 feet. It is impossible to hit a ball pitched from behind if you open up....Good Luck