Cat chewing off his fur at 17

Question
My purebred male Siamese Tai has started to chew off the fur on his back for no apparent reason. He's never been outdoors a day in his life and doesn't have fleas. He is 17 years old but very spry and playful as a kitten and is at normal weight (9 pounds) is afebrile and nothing has changed in his diet or routine at home.
He has never done this before and he is chewing off his fur at a remarkable rate. He is neutered and has been since 6 months of age.  He's never been sick ever and I've had him his entire life.
We started feeding him Iams kitten food as a supplement to his other food since he is a geriatric as I felt a little more nutrition might benefit him.  He had lost weight but the change in food put the weight back on and he is a free eater and eats dry cat food and has for his entire life.  I'm convinced the dry food prevents tooth problems as he has very good teeth.
I don't know what to do about this hair pulling problem.

Answer
Any cat that is pulling out that much hair is most likely itching. Some older cats (and people) develop food intolerances when they get older.  If there is something in the kitten food that is making him react, then withdrawing it for 6-8 weeks should tell you.  There are many indoor allergens that can affect a cat...not just fleas.  Sometimes, itchy cats will respond to an antihistamine called chlorpheniramine.  Ask your vet if your cat can try some.  If all else fails, a shot of a long acting steroid called DEPOMEDROL should give him some relief from the itching and should allow hair to re-grow.  The shot can be repeated avert 3-4 weeks if needed.