Licking Fur From Tummy and Inside Legs

Question
My male cat (4 yrs)began with a small patch on his tummy where he licked the fur away.  The vet said it was a habit and seemed to indicate the cat would stop.  It hasn't.  He now has all his lower tummy fur licked away as well as the inside of his legs (front & rear).  In a recent trip to the vet, I was told that he may have a food allergy and to put him on a Limited Ingredients diet for a month.  He wouldn't eat this diet so was hungry for days and, then, began to vomit.  I couldn't bear this any longer so put him on a grain free diet (Origens) which he prefers, especially the kibble.  He has always been fed a quality food (kibble & canned)so I don't think the issue is the quality.    I've also tried the raw food diet which he would not even consider.  Since he developed this shortly after his adoption as a kitten and that it's progressing, I've considered an allergen within his environment.  I've eliminated everything that I thought it may be down to the litter.  Is it conceivable that he's allergic to it.  I use Arm & Hammer Essentials.  I contacted A & H for additional input re their ingredients but got nothing useful.  The product is corn with baking soda and a fragrance. I've used this product in two living locations so, to me, it makes sense that it could be the litter. I'm concerned about changing to a clay litter because of the dust and that my other cat rolls in it.   Would appreciate greatly your input.

Answer
This is called barbering and it is most often related to itching from allergic skin disease.  It is sometimes difficult to tell what the cat may be sensitive to even with allergy testing or with diet trials, so there are two things we do to see if it truly is an allergy.  The first is to use a drug called chlorpheniramine orally for 2-3 weeks to see if it helps.  If not, then we give a shot of a long acting steroid called DepoMedrol.  If neither of these helps, then we think that it is a psychological problem and we use a drug called Elavil.