You're not saying the step-son is a problem himself. It may actually ease the pain if you welcome him and get to know him as a person your husband cares about. Separate out the boy himself from the pain of the past.
Also, think of it this way: The boy doesn't have a great father. Would you want to refuse him the chance to connect with someone he sees as a good father figure?
And if your husband in turn cares about him, would you really want to keep your husband apart from someone he cares about?