Sorry to hear he is still having pain. I would search out the best Sports Medicine Doctor in your area to get a second opinion. Preferably, one who has extensive work treating pitchers. Not all physicians are alike when treating sports injuries.
For instance, my daughter was playing college volleyball and complained of her back and buttock area hurting. She was advised by doctors that her back was having spasms and she needed to rest. After two weeks of rest, she began working out with the team, and again felt severe pain in her back. I brought her back home to have her checked out by back specialists that treat our NFL team and College athletes. She was immediately scheduled for an MRI that revealed three herniated discs. One was impinging on a nerve. Unfortunately, after physical therapy, she had to have surgery to relieve the pain.
My point is is that getting a second opinion from a specialist that treats pitchers may be your best bet.
Good luck and please keep us posted on the outcome. Any information provided could possibly help other young players in the future.