A son's tooth may grow behind a baby tooth for a few different reasons. The most common reason is that the adult tooth is coming in at an angle and pushing the baby tooth out of the way. This is called crowding and is a very common occurrence. Another reason may be that the baby tooth was not shed when it was supposed to and the adult tooth is growing behind it. This is not as common, but can happen if the baby tooth is very stubborn. If you are concerned about your son's tooth growth, please consult with his dentist.