Quoting *Gaven & Ashton's Mommy*:" Thats a yucky situation. But good for you for being honest with both of them... Did you have sex with ... [snip!] ... and develop a relationship. You dont want to rush and you want to make sure he wants to be with you other then for the baby."
" It would be hard if he got attached and the baby wasnt his "
Maybe,maybe not. Depends of the guy, honestly. I got together with SO when I was 8 weeks pregnant with my Ex's baby. My situation is a bit different since SO and I had been close friends for three years prior to me being dumped when pregnant. SO was really attached to the baby and fairly well knew it wasn't his. When I lost my son at 14 weeks, he was devestated. He was already seeing himself raise that little baby with me( even though my ex was involved in the pregnancy and wanted to be involved too after the baby was born.).
In OP's case, the ex doesn't want to be involved and doesn't seem to care...so if ONS wants to be there for the good reasons...it shouldn't matter if the baby is his or not biologically.
That is, of course, just my opinion based on my personal experiences.