A-HA! Well, this is all starting to make sense then!
The incessant licking of that area is very suggestive of something called feline hyperesthesia syndrome (FHS).
With FHS any breed can be affected but Siamese, Burmese, Himalayans and Abyssinians are the most common. It often starts when the cat is between one and four years of age.
Before your cat can be definitely diagnosed, your veterinarian would need to rule out a few other possible causes for this behavior. These would be fleas, seizures, allergies and bladder inflammation (some cats behave like this when the bladder area is uncomfortable).
There is no specific test for FHS. Instead, it is a "diagnosis of exclusion" which means that we have to rule out other problems first. It is sometimes triggered by a stressful event (a move, a new pet, a new roommate). If you look back to when this FIRST started, was there a stressful event? A visit to the vet for some illness, a move? And more recently I would wonder if your cat may have experienced unusual stress? A move? Renovations?
Treatment can be difficult and varies from behavioral management (tapping him on the hind end when an episode starts to distract him) to medications. Some people have had success with acupuncture.
Treatment also involves trying to minimize the stress in your cat's life. One of the things that I would strongly recommend is a Feliway Diffuser system. This is a plug-in device that sprays a cat pheromone into the air, helping to calm the cat without drugs. Here's a link to more information:
I find the plug-in diffuser to be the most helpful since it works even when you are not there, which is when your cat is most likely to feel stressed.
If this does not work, your veterinarian could prescribe a number of different like drugs for cats. As with humans, sometimes it takes a while to find the best medication for relieving your cat's stress and you might have to try 2 or 3 different drugs before finding what works for your cat. Usually, medications are only used when the problem is severe.
For more information, here are links:
So, in terms of what you can do for your boy, one very easy thing would be to treat him with Advantage in case he has a couple of fleas driving him crazy. I admit that is not likely since you haven't seen any evidence but it is so easy to treat him for that it's worth doing so we can at least cross that off the list of possible problems.
Also, try distracting him when an episode starts by tapping him on the back end, or squirting him with a spray bottle. This may stop him in his tracks!
Also, you could take a video of him having an episode. As you can see from the links I gave you, videos are worth even more than one thousand words! They really are helpful to diagnose FHS.
Lastly, a source of Omega 3 and 6 fatty acids can help to decrease inflammation and dermatitis. I would suggest that you consider a dietary source of essential fatty acids (Omega 3 and Omega 6 fatty acids) daily added to the food.
Your vet would be able to provide these for you - some brands are DermCaps and EFA-Z.
Here is more about them:
Good luck with your boy!
I will still be here to provide more information if you need it!