She refused to give me a DNA test.
You're not entitled to one, and have no standing to pursue such in court, either.

I have asked her to leave him alone and she keeps sending him messages on FB and calling his friends.
You need to be having a conversation with your husband. If the child is indeed his, he's going to be in contact with this woman for the next 18 years, at a minimum, and you'll need to reconcile yourself with that fact.

Alienation of affection suits are notoriously difficult to win, and the state of Illinois is rather hostile to them to begin with.