What could be wrong depends on where he's licking. Have a look and try to see what part he's licking. Is he licking his weewee? If so, he could have a bladder infection, and could be on his way to having a blocked urethra (very dangerous!) If that's the area he's worrying, get him to a vet as soon as possible. It doesn't take long for the developing cyrstals to block the urethra (been there, experienced that!), and I'm told that can result in death if not treated!