Our dog had the same problem and it was his food. He was allergic to it. Your dog quite likely has an allergy to it 's food. The top three allergens for dogs are corn, wheat, and soy. There are other possibilities but these are the first that should be checked.
He might be feeling itchy on that portion. As you know dog scratch with their tongues. Check it out whether he has any skin problem. Or he might have developed a habit.