how to move on when my fiance has another baby

I have been with my fiance over 15 years. We have ended our relationship twice within the 15years. During our last break up for about 4 months back in 2010 I started to see my ex-boyfriend. My fiance started to see another woman too. We got back together and stop seeing the other people but at the time, he didn't tell me about the woman he had sex with a few times. We love each other a lot! I am close to his family and we have two kids together. He just told me the other day during our break up, he was with this other woman but she got pregnant and she had his son. My fiance has an older son also whom I love and he was there before me. The woman who had the new baby does not want anything to do with my fiance, in any way. They didn't have a relationship, they just had sex a few times. She really don't care if he sees the baby or give her money for this baby. This is her only child, plus she was told by doctors she could not have kids and this is why she kept the baby because it may have been her only chance to have a baby. Now, I would like to know should my fiance and I work this out? He loves me and I know he does but, I wanted to have his son and I don't know what to do now.