What should I do about my husband's son?

My husband and I started dating when we were 16. We broke up when he was 19 for a while in which time he got someone else pregnant. He did not have any contact with the child and we ended up getting married. We have two kids together (a 3 year old daughter and a 5 month old son). The boy is now five years old and my husband says he wants to be involved with him. He has told me since we got married that he didn't want to see him so I thought I would never have to deal with this. I love him and want to be with him, but I also feel like I can't accept being a stepmom. How can I make this easier? I feel like this is much more complicated than the normal stepmom scenarios because of these cirstances.