I am a good friend with my BF’s daughter. When I met her first time she was a little girl but now she is an adult - 18 years old. I choose not try being mom to her, she has real mom around her anyways.
I had mixed feeling at the begining. However time goes by... I love my BF including the way he loves his daughter. And I love his daughter very much - thanks to her, she opened my heart I guess.
I met her mom (Bf’s ex) several times - first time she was trying to show that he is still her man... but again time goes by, she has no more attitude like that. She is a good mom for the daughter I respected her.
My parents did not have such an experience so they do not understand that I am happy like this... this is the hard part for me still. But again time goes by... I hope so!
I am sorry for my bad English, there is no Japanese here but I see myself 10? years ago on your message.
I think... You and your boyfriend should meet his ex and child once before talk about your marriage. His ex and son will not disappear from his life so you guys should find them, face to it.
Even he thinks there are less than 10% possiblility that this kid is his, he should have a feeling for his may-be son and so as his child. Boys need father when he grow up, father far away from his boy should not be a good man.
I think you are lucky ones who has given a chance to learn very important something about life this way. Find out what you fear so much... then you will know what to do next for sure.