Thumb pressure and it’s effect on location

I’m interested to hear anyone’s take on varying thumb pressure as it relates to location. My son is 16 and has developed several pitches - 2 seam FB, 4 seam FB, CB, slider, circle change - and seems to have index/middle finger pressure figured out. He has an absolutely wicked CB, but it is very erratic in command. I asked him all sorts of questions about grip, arm action, doin, etc and the one thing that stood out to me was that he didn’t seem like he put a lot of pressure on his thumb.
Now when this thing spins off wildly he could hit the catcher in the next pen over. My thought is that he does this when he gets more 3/4 and because of the loose thumb he doesn’t hold the ball well in his hand, and it flies off at a random time and release point. When I get him more over the top it’s far more accurate, wondering if it has anything to do with him maybe gripping stronger like he would with his FB, or just because the thumb is now on the bottom rather than off to the side a bit, therefore it doesn’t “hold” the ball in place as much.
Any thoughts?