You sound pretty nervous about it. I think that yes, you should tell him about the nevus. Tell him the truth that you are nervous/anxious about telling him because it is a part of who you are. If he loves you, as I suspect he does greatly, he will be very open to hear you out. As it is, you then at some point should show him. He will be curious to see it. Don't be afraid or ashamed. This is how God made you. You are exceptional...your fiance know this and will accept you for all of who you are. Don't try to hide it. Just come right on out and share it. When your fiance sees that you have been worried about this, he will most certainly tell you that he loves you no matter what, nevus and all. You will be fine, honey. Just gather your courage and let it all out. Blessings to you from Oregon.
You need to tell him