Well, any time there's a change in behaviour like this, it's wise to consider a vet check up, to rule out neurological issues, Cushings disease, etc.
Is he a bit nervous, or prone to stress? I have a Rottie who is...she will lick her feet, chops, furniture etc when she is stressed. If there's nothing medically wrong, consider the stress (or boredom) angle, and go from there.
Cooper The WonderDog
Daphne The Destructo-Rott