Sounds like your cat might have Cheyletiellosis (aka "Walking Dandruff"), which is caused by mites. It's probably a good idea to get him to a vet as soon as possible because it is highly contagious (and can be transmitted to people). You'll also need to treat the environment, because if they're on your cat, they're also going to be in his bedding and around your house. Here's a link:
http://www.petsandparasites.org/cat-.../dandruff.html