You can also get him doing self-assisted shoulder flexion if you teach him how to do it correctly.
Try active- assisted movements rather than passive movements. As he has no activity get him to imagine the muscles contracting as you're doing the movement. If he finds this difficult get him to move his right arm simultaneously so he can use the sensation of the movement in his right to imagine how it would feel in the left.
How is his sensation? If his sensation is good - assist him to handle different objects with his hand to provide stimulation.

Let us know how you're getting on.