I just got on this site. As an ex Olympic Lifter and who has a son who is a whiner and who wants to be a pitcher, I have probably seen every overuse shoulder injury there is.
The most common things that are missed by MD’s and PT’s is a mild shoulder separation, and a subluxing bicipital tendon. Also look at an anatomy book and look at the insertion of the deltoid muscle. They insert lower down than you imagine. Pain in this area is a deltoid dysfunction.
If you tell me where you live, I might be able to give you the name of someone who can help.
That being said, where exactly is his pain?