Find someone who teaches him how to take care of his arm - both in terms of mechanics and conditioning.
Find out who are the PC's pupils. Ask around. See how many of these boys are having or recently have had arm problems. Watch them a little, see how their mechanics look.
Don't commit to more than a couple of lessons initially. See how he works with your son, how your son responds, etc.
IMHO, the good PC's will just start working with the boy. They will work on one or two aspects. It will be a low key approach.
The shysters will make a big issue of how bad your son's mechanics are, how he is in mortal peril, but he - the great PC - can fix all the many faults given enough time and turn your boy into a MLB pitcher. Their ego will show through. Run, don't walk, away from these types.
Once you settle on a PC, please be sure to do weekly lessons on a regular basis. Sporadic lessons are pretty much worthless.
If I am saying things of which you are already aware, my apologies. Not having a knowledge of PC's in that area I can't answer your specific question.
There are many shysters passing themselves off as PC's. But precious few good PC's who can teach.
Good luck.