Even if you don't physically see any fleas or other external parasites on your cat, one flea could have bitten him and if he's allergic to flea saliva, he could have this fur-pulling reaction after the flea is long gone. This is called Flea Allergic Dermatitis or "FAD." You didn't mention if the vet prescribed any medication for a flea allergy.
If you treat him with a monthly topical flea preventative sold by your vet's office, this will prevent him being bothered by any possible fleas in the area. If this area on his back is itching him, you can give him an over the counter (or one given by the vet) antihistamine like 1/4 of a 4mg. Chlor-Trimeton every 12 hours and see if this helps.
There is a condition called feline psychogenic alopecia, where cats will lick or bite at certain areas of their body until it is hairless. This is typically attributed to stress or sometimes it is 'idiopathic,' meaning it has no known cause. Here is more information:
Another possibility that you can discuss with your vet is feline hyperesthesia syndrome. This causes the cat's back, where you described, to be very sensitive, the skin may twitch and 'ripple' and the cat will bite at the back (and often, the tail, too), frantically, and pull out fur. Sometimes, cats with this condition will run around the house as if being chased, all of a sudden, even from a deep sleep. This condition has been likened to seizures, so your vet may want to do a neurological work-up or refer you to a veterinary neurological specialist.
These symptoms may also be caused by a food allergy, so make sure your cat's food does not contain any soy, wheat, gluten, corn, careeganan, artificial flavor or colors and/or preservatives. Grain-free food is best and so is canned food. Here is an article written by a feline vet:
There are some limited ingredient diets that contain novel proteins and carbs like duck and green pea, venison and brown rice, etc. Many manufacturers make these, like Royal Canin, Natural Balance and Blue, plus, many others. You can find some in your pet supply store and others at your vet's office, or they can order it for you.
I hope all will be well with your ginger kitty!
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