I have seen a few babies who have this. It is similar to tounge tie if you have ever heard of that. We call it a "tight" upper labial frenulum although I'm going to guess that's not the medical term, lol. I really only know how this "affects" the baby from a breastfeeding perspective but normally if he is gaining fine and mom isn't in pain there isn't an issue. It is my understanding that it recedes on its own eventually. With tounge tie there is some indication that it could cause speech problems later in life but I'm not sure if that would be the same issue in this case. At any rate, I think you are right that there is no reason to be alarmed. I'm sure your doctor has seen this sort of thing a lot.