From what you have said, I wonder if he may have a posterior osteophyte causing some irritation, in which manula techniques to the C spine may not provide much in the way of long term relief.. I would look at myofascial trigger areas in the region, esp Infraspinatus, which is reported as provoking paresthesia in the arm, and is innervated by C5/6.
Long term management will probably involve a self management programe of postural re-ed and exercise I would have thought, hope this helps