Please help - cat licking off hair and vet cannot give us a reason why!

My six-year old cat started licking off his hair on his back last summer, and it has gotten worse ever since.  Now his back is bare and so are his legs and paws.  We cannot think of any reason for why this started last summer (no change of food, we did not move or introduce a new pet, etc.)  The vet tested for mites and ringworm, we gave him flea medication, and we did a few months of a hypoallergenic food that the vet suggested and nothing seemed to help. The vet also ran an allergy test, but the things that our cat is allergic to were seasonal allergies which makes me doubt that they are the cause since he has been licking all year long.  Our vet does not seem to have any clue, and I feel horrible since our cat seems so unhappy.  Does anyone have any advice?  We are really at a loss!

Do you feed him only canned food, high (animal-based) protein, no grain, low carb?

And do you provide him with filtered water, not tap water?

Where are you located? Are there any holistic vets you can see - trained in chinese herbs? If you're in the U.S., you can search here: http://www.ahvma.org

When we rescued our Chumley, he began obsessively grooming after a few months on a cat food diet (he was a feral cat). He mowed his abdomen clean, and spots on his forelegs. We did the same things, and nothing helped. We saw a holistic vet, who got him on the right track immediately - with the diet change and the herbals she prescribed.

LDG has made a very good suggestion.  In the meanwhile have you tried covering him up with a doggy outfit? Is there a cat antihistomine you can try?

One of my sister's cats does the same thing.  The vet couldn't find the cause and said it might be OCD but didn't offer any solution.  Many years ago one of her other cats would urinate around the house and was given an antidepressant for a short period of time and it worked.  I forget the name but if my sister remembers I'll let you know.  These medications are used for OCD also.  She's going to ask her vet about it.