Check the neck fully, correct and protraction of the head on the shoulders. Release the supraspinatus in the fossa etc.
Check for all neuro first though and clear for mm involvement/weakness, reflexes that could indicate nerver root involvement. Perhaps tape to inhibit the upper trapezius as a test to localise the problem area. If he is not using a cordless, wireless mouse then suggest investing. This does give more freedom to the hand and arm.
Keep in mind it is no uncommon to give youself a rotator cuff tear or nerve traction injury in the axila while sleeping. Ask about how he sleeps, pillows etc and correct as needed to unweight the head on the shoulders while symptoms are persisting.
Look forward to more comments on your assessment 8o