I have a 10 month old Cardigan and I noticed today that on one of his back teeth on the outside surface he has a black spot. It's about the size of the tip of a ball point pen. HIs teeth look very clean and white so I don't know what this spot could be. My first thought was cavity but with him being so young and his teeth being so clean it seems unlikely. He won't let me brush his teeth but we make sure he has plenty to chew on to keep the plaque off. Any ideas?