My son has a similar issue but may have been caused by a different issue than what your son is feeling. He doesn't pitch but at one time his throwing mechanics were flawed and he actually "pushed" the ball rather than throw it. This caused pain for him when throwing the ball with a lot of velocity. Turns out he had damaged the tendon that attaches the tricep muscle to the elbow. The tendon began to "splinter" and caused the same type of pain. Same scenario though - light throwing was fine but when he picked up velocity it started to shut his arm down. It's now all fixed (mechanics) but the doctor recommended ice and Aleve before we noticed the throwing flaw.