His dentist said that unless it causes any issues to leave it until he's older.

Does it affect how your baby drinks at all? I would ask their dentist to look at them when they go and see what they think, some will not do anything if its not messing up their teeth or latch as a baby. My son had it and one day we were at a restaurant and it just split (I'm guessing he hit his face while I wasn't looking). A lot correct themselves when a child falls and it tears.