Mole on child's face

My son is four years old and has a mole about the size of my pinky nail on his cheek. He was born with this. It is a raised mole, and already, children have begun making fun of this at school. He told me this morning that his friends told him "it's ugly" and "not pretty". Was is the best advice and/or thing to tell my child to say to other students when they say this to him? I have already told him that he is special and his mark is special, but what should he tell the other kids?
Thank you!

[Kevin Kennedy, Ph.D.]

This poses a challenge because of his young age. A rote reply would be best, because he will more likely be able to remember it if the situation arises. I would lean in the direction of a comment that normalizes his situation, so something like: "It's just one of those things; I'm OK about it."  Maybe some of the reraders of the Forum will offer you their thought.