It would be worth having your vet examine him to be sure that none of these other problems is likely. Flea allergy is particularly important to eliminate. In several studies of cats with this condition no fleas were found on exam of the affected cats in a majority of the cases. Apparently, cats are very efficient at removing fleas from their bodies if they are allergic to the bites and irritated by the flea. If this is not a physical condition, it is sometimes beneficial to do things like grooming or games which distract your cat.
Antihistamines seem to help some cats even when the problem seems to be psychogenic.
There are several anti-anxiety medications which can be helpful and it is often possible to use them for a relatively short duration of time then wean the cat off the medications. Sometimes, just breaking the habitual cycle of licking and chewing helps for a long time. I took her to the vet in January and they ran extensive tests on her and found nothing physically wrong with her. She was given steroid shots to possibly stop the psychological licking, but that did not seem to work. Some cats lick their abdomen when they have cystitis or other causes of abdominal pain.