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?

Hi. I’m old now, but back in 1969 I played varsity baseball and was picked from tryouts as a pitcher. As I remember and still do, my thumb was always tight on the ball.

The fingers have different impact on the balls’ rotation! I had various success by pitching with different grip locations and strength.

One additional factor is the wrist strength. I found that many coaches didn’t mention or teach about wrist strength and flexibility.

Infielders and outfielders have different requirements how they use their wrists in catching the balls!

Just my 2 cents