My child's father is my cousin

The Jamaica Star has an advice column called "Tell Me Pastor" and a 27-year-old woman wrote in asking how to explain to her current boyfriend that she had a child with her cousin 10 years ago:

I don't know if he would believe anything I would say. I spent a whole week with him when he was on holidays. He took me to his parents' house and introduced me as his girlfriend and his mother likes me.

Pastor, I am so confused. I had my first child when I was 17 years old and her father is my cousin. So you see I am afraid to tell this man the truth. My second child's father supports him, but he lives with his grandmother.

I know when he hears the truth that that would be the last of our friendship. Should I leave this guy, or should I tell him and ask him to forgive me for not telling the truth earlier?