Why not let him hum? Isn't that a happy thing for him to do? I used to hum a lot as a kid, before I became good at whistling and later learned to play some instruments. Do you think he'd be interested in learning more about music?

If you're worried that it's a sign of a disorder, I would just ask his doctor about it.

Striving to increase my rate of flow, and fight forum gloopiness. sick