Partner has a child to another? HELP'

Ally
Okay so, me and my partner have only just recently gotten together but have spoke for a full year.. We are currently trying to have our own baby
BUT there is one thing in our relationship that is a big bump in the road... 
So in may a friend of my partners rang him and said she had just given birth to his son! We didnt even know she was pregnant to his child, so it was a big big ahock to us both!! 
I have not met said child as it makes me sick that he has a child to a woman he doesnt love nor want to be with, we got into a huge arguement tonight because i told him i refused to have anythingg to do with his child... He told me i am this childs step mum and i said no, one thing lead to another and it ended up in a MASSIVE argument!! 
Im just confused and hurt im not sure if im being selfish or is what im doing right? Please help!