Have you got the InSiminator installed? If so, try to 'restore' him with that. I can't quite remember what part to click on but you'll find it after a bit of poking I'm sure... The InSiminator fixes a lot of weird glitches like this. It couldn't really hurt, I don't think.
Incidentally, the above referenced Michael Jackson condition is called vitiligo. In people who have this condition, parts of the skin just don't do pigment. It's a bit rough for people who have it, since the unpigmented skin burns very easily and they have to be very careful to protect it, and also they can appear blotchy, which makes people look at them funny. Personally, I think it looks pretty cool. If you're interested in having sims in your game with this kind of look, do please go here:
Genensims.comThere's a lot of excellent work on this site, and some very unusual skins!