this is typical if he fell in abduction and had a distal force applied to the elbow. this stretches the ? medical fibres of the inferior scapulohumeral ligament/capsule. You see the alot in baseball pitchers and tennis players mainly from the wind up to pitch or serve. For them it is often a lack of thoracic extension or the use of it in the movement action. Jenny McConnell and Craig Allingham (in Australai) do a great workshop on this type of injury