I've been through this so I can tell you exactly what I did.
About seven years ago I met a great guy. I had two kids and he didn't have any. About a month after dating I found out that he had a baby on the way. I was moreso mad about him not telling me about it than anything else. I mean if I have two kids it's not like I'd be unwilling to accept the fact that you had one. And I was mad for him being an adult and not using protection.
He said he didn't want to tell me because he wasn't sure if the baby was his or not. Still he should have let me know.
Anyone we stopped dating and just became casual friends. I told him that I liked him but didn't want to get too involved until all issues were resolved with he and the other girl and I knew that wouldn't be until at least after the baby was born. I told him that I knew him well enough at that point to know that if at all possible he would want to be with his child everyday and be with the mother of any children he had.
Well she had the baby, he was there for her, but they never developed a relationship and just coparented. And that worked out okay. We started back dating about 6 months after that and eventually got married. Still happily married today.
I urged him to get a paternity test after the baby was born, but he just never did. He finally decided to get one about three years ago. The baby was/is not his.
We still talk to her on the phone (we live in a different state now) and see "his daughter" when possible. She still doesn't know DH is not her dad and we're okay with that for right now. But that's getting off into another subject.
Anyway I'd evaluate why they broke up in the first place and be absolutely sure that relationship is over and done with before going any further.
I only liked him at the point that he told me and definitely wasn't in love so it wasn't a big loss to me anyway when I told him to work on the relationship with her first.
Good Luck.