
Hi,
If you're certain there's nothing wrong in that area, ie full anal glands, normal poo, no sores or anything else that could cause this, it might be a "habit". Sometimes dogs keep on doing certain things, just because they usulally do them :p If your boy did get a bit itchy originally, he then maybe kept on licking himself out of habit? Especially if the skin is getting a little irritated from his licking, it can become a bad circle, that needs to be broken. Try discourageing him from licking, and if he still can't resist it, even when told not to, an elisabethan collar might be needed,.
This is only in the event of you being absolutely certain there's nothing wrong. If it's been a while since the vet checked him, maybe a new visit at the vet's should be undertaken, before you actually hinder his licking.
Giving this kind of advise over the internet is always difficult, and a new examination by a vet, before you do anything would be my advice.
Good luck,
Karen