External parasites can be a possible cause, with fleas being the most common. Even the bite of one or 2 fleas can set off a cycle of irritation. There is a mite that can infest cats, but they aren't as common. You most likely would see bumps and crusts on the area on his back. Your vet can do a skin scraping to check for these mites. Ear mites can migrate to other areas of the body but you most likely would have ear problems too.
They may have sensitivity to something in their food or environment. Has there been any changes in diet or the environment (shampooed the carpet, new air fresheners, new detergent)?
Some cats will lick excessively in areas of discomfort, like when they have arthritis or a bladder infection.
When all medical reasons are ruled out, there is a condition called feline psychogenic alopecia, where stress factors may cause them to overgroom. Any major stresses going on at home?
Here is a link to info on psychgenic alopecia, which also mentions some things to rule out when coming to a diagnosis:
http://netscape.petplace.com/Articles/artShow.asp?artID=3148
Holistic Wellness Care