“ I believe that my son is pronating his forearm way early in the delivery.”
If you mean when he starts the ball forwards then he is on the right track. Forearm drive pronation eliminates many Elbow pathologies !! The problem arises in understanding how to arrive at the back that matches the next action.
If he arrives traditional with his forearm pronating on the way back and up he must correspond with an intuitive forearm supinated drive start. It’s intuitively antagonistic for that action.
If he is going to drive the ball with forearm pronation he must arrive at the back with forearm supination, now this action matches intuitively for forearm pronated drives that actually match what the Humerus has to do, inwardly rotate also.
“ Its bad enough that his coaches and his teammates notice it, because it just looks weird.”
I would say it’s much worse they are all taught an injurious arrival and most of them will miss the opportunity to continue because of their pathomechanical ignorance.
Best way to describe it is that it looks like he is trying to snap off a screwball!
He should !!! It’s the safest pitch on the arm. Have you noticed in the last 5 years the proliferation of the fastball Sinker and turning over the changeup? The science says it’s the way to go.
“He has some nasty ball-side movement”
Want to see it improve? Ask him to throw from a high arm vector, stay tall and rotate off the glove side leg. Ask him to bring the ball back the up by supinating in the way back.
keep him away from forearm supinated pitch types like the ¾ S.Curve, S.Sliders and S. fastballs.
“ he does complain of pain in his upper arm, which he didn’t typically before.”
Exactly where? If his forearm traditionally flies outwards and he’s trying to pronate his forearm, the centripetal force of his Humerus going one way (traditionally arriving outwards) and his forearm going the other is this out of sync problem that can be easily mitigated by corrected arrival and understanding between by arrivals at the back
“Towel drill maybe?”
If you want to learn how to snap a towel and really bend your back, do this one.
“Any drills or cues I can use to help him out here?”
The best drill in throwing or pitching by far is the “no stride drill” where he puts his ball foot toes over the front lip of the rubber, puts his glove foot out in front with the foot turned out and in it’s leg lane on the glove side of the field driveline about a power walk distance (short) with his glove and ball together at the belt.
Have him pendulum swing his glove and ball simultaneously down then back then up by having the ball arm supinate on the way back and the glove arm pronate on the way forwards. When he arrives to length with the ball back and at driveline height (top of head high) immediately pull back on the glove arm by supinating the forearm against the glove side pec. and raising the ball elbow up. When you drive the ball pull back against the glove side leg by extending it fully, staying tall and rotating the Torso 175% by trying to end your ball side hand in near your back pocket.
Hope that helps, any questions feel free.