A.T.
How is the woman? Is she willing to accept you and does she want her son to know his father? I would suggest thinking about it in this way - giving her the benefit of the doubt - did she know he was just separated and wanting to come back to you? Does she still want him? Don't look at the baby as a bad thing b/c it was something your husband did when you were separated. I know that is much easier said than done, however, you CAN love the baby. Kids of parents who are no longer together CAN thrive if they see that ALL parents (including step) can work together.
Now, after I say this let me tell you that I live both worlds. I have a daughter who is 14 and just moved in with her dad and stepmom. THEY are WONDERFUL, and some of our close friends. We all work together and have a relationship most don't think is possible.
On the other hand, we have full custody of my husband's two daughters. The "mom" is insane - no lie. He won custody four years ago and after constant court battles (which I do not suggest!) she was ordered sup'd visitation and now doesn't see them at all. They have adjusted very, very well, and are doing better without her drug and alienating behaviors.
I told you that so you'd know that I do see both sides. If the mother is not easy to work with, things can get harder. But, talk to your husband (soon to be) and see if this is something you can both handle before you get remarried. Otherwise it could lead to another divorce. Just don't ever deny him the right to see his son, in fact, encourage it. Think about it this way - he is a baby, you will know him since he's tiny and be able to form a bond with him without his having future knowledge of anything. I hope that makes sense! Good luck!