I (F28) just found out my bf (M28) of 18m has a child and is pushing me to be a mother to them. How should I proceed?

I just found out last night that my boyfriend of 18 months has a 4yo child and is expecting me to mother the child. I am absolutely not okay with this, but I have no idea where to proceed. I love him dearly and he knows how much I love him, but I can't do this. I am really devastated because he knew I never wanted kids and that having kids would be a deal breaker for me. Up until now I had no idea the child even existed until weeks ago when his current baby mama threw a tantrum on social media about how I am the reason their baby is suffering from not having a dad. <br><br>So now we are having this huge fight and he is trying to guilt me into being a mother to this child. I have zero intention of doing so. I have already spoken to my mother and she is furious. I'm so conflicted about this whole situation. I feel betrayed and lost. What are the next steps I should take in this situation?<br><br>[UPDATE] I did end things and he tried to get me to marry him. I have moved on and am very happy with my new man. Thank you all for the advice, it really meant a lot