Thank you for all your helpful replies. The patient has since had an orthopaedic consult who has advised him that it is not a surgical issue, apparently he has seen "loads" of unstable SCJs and that it is not as unusual as we had suggested (my senior has 25yrs MSK experience and has never seen one). He has told the patient that he can continue with all sports and that symptons should relieve in time.
This leaves us in a situation where the young patient has NO desire to continue with an exercise programme as the consultant did not specify directly that he should, even though he is still experiencing the same levels of pain.
To answer some of your questions, yes there is evidence of generalised ligamentous laxity. And altough yes we feel it would be beneficial to put the arm in a sling and immobilise for a couple of weeks this is not possible as it would restrict his school work hugely and keep him out of competitions, we know that he would not be compliant and it is unlikely we would get his parents on board unless we could promise results. Also because the orthopod was almost dismissive of the problem we feel that our opinions and advice may fall on deaf ears!

Thank you for all your help and if I'll keep you informed should we get a breakthrough !!!