Support the weight of the arm, perhaps with him sitting and with the arm supported on a table,pillows etc. The do some deep tissue work along the supraspinous fossa. Supporting the arm allows the upper traps. to relax thus giving you more access to the deeper supraspinatus muscle. On could then try taping down the traps when they are in the relaxed position to "inhibit" their action somewhat. I have found this very useful in acute shoulder joint issues with referred arm symptoms.