Not being a doctor, and never having seen him in action... there are typically two things that will destroy a shoulder in this biz.
- Using your arm to push the wand instead of using your lower body. As a tall guy that did giant cleaning jobs where I worked 14 to 16 hour shifts, I learned the value of standing up straight and stepping back and forth rather than leaning forward and shoving the wand with my arm.
- Swing the wand with both hands. All kinds of bad things build up when you repetitively move one way all day, and changing hands while working feels like taking a break.
Our guys don't get out of training until they are competent cleaning with both hands and lose the habit of overreaching, which leads to bending forward, which leads to back aches and rotator cuff surgery.
I'd suggest starting the process by learning to clean with his off hand, and while still clumsy anyway, start by cleaning with the thumb of his wand hand stuck in his pocket while standing up straight. Once that feel sets in he can take his thumb out of his pocket and let his arms swing a few inches forward. Takes a while to become natural.