I love my fiancé to death and it sucks that he cant understand why I don't want our son around his…

I love my fiancé to death and it sucks that he cant understand why I don't want our son around his mother when he's born. The reason behind it all is, his mom tried to physically attack me not once but twice while me being pregnant. Also she keeps on trying to get him to give her money all the time and we barely have enough and need to start getting more stuff for our baby. But she doesn't care it's always about her and what she wants , she could care less if we and our son suffers from it. She blames me for all of his problems. I just don't know what to do.. I cry about this all the time.