And it is not a cutter if you supinate to the outside of the ball, it is a misthrown pitch and leads to excessive stress on the elbow and shoulder. A cutter is what I call a “mature” pitch, meaning that it takes a certain amount of strength and manipulation and velocity in order to work (Which is why you usually see it developed along with a real change, late in high school or early college). It is usually overbalancing a fastball grip which gives it the break, a slider has a similar action but using extreme supination though it gets movement is a trip down the wrong path.
The methodology Roger and I refer to is breaking down the actual fundamentals of the throw, breaking it down to the basics means you can rebuild more rapidly to a fundamentally sound mechanic, if the stripe doesn’t work, break it down even more, he’s young enough, have his “warm-up” consist of very fundamental throwing routines and gradually build during the warm up. Repetition will be your and his friend, if you train in a fundamentally sound manner.