Given what you describe, he could have medial shoulder laxity:
http://www.vetsportsmedicine.com/resourceCenter/documents/VOSMNwstrFallFINAL11.20.08.pdfBut OCD of the shoulder should also be ruled out, especially given his age
You need to go to an orthopedic specialist for a valid diagnosis. A shoulder abduction test can be done by a qualified orthopedist, but your regular vet will not be equipped (and has not been schooled in this or other diagnostic measures). X-rays could reveal OCD, but wont detect medial instability. If the shoulder abduction test is not conclusive, then an MRI is usually the only other definitive tool.
The OFA (panel of radiologists) grades shoulders and the shoulder joint and this is a good route to go for the cost, relative to a lot of testing by local practioners - you get a 3 panel assessment (3 opinions) for the cost and so its a good deal.