As I mentioned, my husband is a graphic designer. 

I can't think of any dramatic stories to scare him but it is a well known fact now that repetitive strain injuries can get serious enough that he won't be able to keep working if he doesn't take measure to help himself. Arthritis is a real concern for many artists, musicians, etc who don't take care to prevent these injuries from occurring over and over. I know it scares me straight just to know that if I'm not careful I won't be able to do what I love until the day I die. That might work for him.

Arthritis doesn't really cause numbness. If you really want to scare him, tell him this: Sometimes, numbness is caused by a tumor in the brain. The tumor presses on a nerve that should be giving his hand feeling. Since it is present, his hand feels numb. :P

It worked on my brother. :P

If you want to scare him then you can tell him about the condition that struck Michael J. Fox and Muhammad Ali. There hands are always shaking. I wonder what that sickness is called.

Jittery hands? :P Maybe you could look it up on the net or something. If you think about it, there are actually tons of sicknesses out there you could use to scare your husband. :P

Maybe it is Carpul Tunnel Syndrome?
What are the symptoms of carpal tunnel syndrome?

Symptoms usually start gradually, with frequent burning, tingling, or itching numbness in the palm of the hand and the fingers, especially the thumb and the index and middle fingers. Some carpal tunnel sufferers say their fingers feel useless and swollen, even though little or no swelling is apparent. The symptoms often first appear in one or both hands during the night, since many people sleep with flexed wrists. A person with carpal tunnel syndrome may wake up feeling the need to "shake out" the hand or wrist. As symptoms worsen, people might feel tingling during the day. Decreased grip strength may make it difficult to form a fist, grasp small objects, or perform other manual tasks. In chronic and/or untreated cases, the muscles at the base of the thumb may waste away. Some people are unable to tell between hot and cold by touch.