welcome to health care magic,
the symptoms experienced by your son could be
(1)anterior shoulder dislocation ...the round protrusion is the head of humerus
shoulder dislocation could be due to joint hyperlaxity.
your son has marfanoid habitus ,which predisposes to the problem
(2) Second possibilty is whether the muscles around his shoulder have become
thin and u r able to notice the bony prominence.
this is a muscle disorder known as facio scapulo humeral dystrophy
kindly consult the nearest physician for certain blood tests like creatine
kinase,radiographs of both shoulder jts.
hope u find this helpful