Well first of all, she can't ruin you. Not sure what you even mean by that.
Second, you say that you recently found out yet you also say this happened on Father's day. In my book, 7 months isn't recent. So...did your husband not tell you until much later that she reached out? Or is your sense of time a bit off?
If it's really been 7 months, then why hasn't anyone acted on this news yet? Why is your husband being so passive? He should request a DNA test so that you stop living with uncertaintly. He either has another child or he doesn't, there's no need to keep on wondering about it and leave everyone anxious and unsettled. Tell him to get a DNA test so that you can move forward.
If the boy isn't his child, then there you go. If he is, you'll all deal with it accordingly. If that does end up being the case, then I would suggest enlisting a family counselor to help you all figure out how to best move forward in the best interests of this boy, who will be far more confused, fragile and devastated than you are right now. It's OK to be overwhelmed, but don't forget that there's a little boy out there who is possibly feeling much worse than you, who doesn't have the benefit of life experience to help him know that this will all work out. This boy needs someone to advocate for him and end the uncertaintly - be that person and push your husband to do the right thing.
If the other woman was married when this boy was born then your husband has no legal rights or responsibilities to the child because paternity is assumed of the husband for a married couple. Even if DNA proves that your husband is the father, the ex-husband would have to consent to any change in legal status because when a child is born to a married woman, the ties of marriage trump biology. Some states are slowly changing these laws but for most, this still holds true.
If the woman wasn't married, then she could take your husband to court for paternity and child support and that, frankly, would be totally fair. If he made a baby with someone, he should have to support that child financially. Whether or not a relationship with this child is good or possible will depend on individual circumstances.
I hope that your husband has the courage to take the next step and end the uncertainty. Just know that no matter what happens, you will adjust and be fine.