Alot of good suggestions about footwork,arm mechanics and arm strengthening stuff. Stay low and do long toss (at his age, he needs to work up to at least 150-180 feet; as he works back in all throws must be on a line).
As far as instructors, I'd start with the catcher guy. If he sees arm mechanics issues, talk to the coach about seeing the pitching guy to straighten things out and getting your son moving in the right direction (catcher guy may be able to fix the problem also).
Don't know what the cause of the problem is but I had a catcher a couple years that had a habit of dropping his elbow as he threw the ball...really frustrated the kid. His release point was maybe ear high and almost looked like he pushed the ball instead of throwing it. Rainbows or balls in the dirt. Sometimes he looked like Johnnie Bench, others like his grandmother. I had his Dad make a stand out of thin PVC pipe high enough that he had to release the ball at the proper height to clear the top bar. The kid spent the winter throwing over it into a tarp in the basement (along with working on footwork and lifting). Saw him the next spring at his HS game and the problem was gone. His velocity was up with alot more accuracy. The kid said he threw about 50 balls everyday, 7 days a week, using the PVC contraption. I told him the PVC didn't fix the problem; he did by working hard over the winter.
Also look at the angle of his throwing elbow after he takes the ball back and before he come forward to throw. If it is less than 90 degrees ( ball closer to his ear than his elbow), he'll have a tendency to throw pie- plates with his hand moving under the ball instead of staying behind it.
Arm strength is important but transfer mechanics and footwork will help him improve his release time so be sure he works on those as well. A cannon arm is wasted if the ball is in the mitt too long.