I can't leave my negative environment without leaving behind my fianc

My fianc and I have been in a relationship for two years. We have an infant son. My fianc also has a child from a previous relationship. We do not live together. I live with my mother currently while I get on my feet, and he's living with some friends. My mother and I have an awful relationship that is completely unbearable most of the time. We cannot even stand to be around one another while living in the same house. She has made it clear that she wants me gone. Recently, I was talking to my father who lives in a different state. My father and I have always had a good relationship. I explained to him the situation I am in with my mother, and he said he would like for me, my fianc, and our son to come live with him and his wife. I would really love to go. I do not want my son to be in this environment with constant arguing and negativity any longer. There is nowhere else for me to stay while remaining in this state. The problem is my fianc is refusing to move away with me because he does not want to leave behind his other child. He and his child's mother have a horrible relationship, and she would absolutely not be willing to let him visit if we moved away. I believe I would be doing what is best for my child by moving away, but I do not want to leave without my fianc. I have already explained the situation to him, and he will not give in. Do I continue to stay in this negative environment with my child and keep our family together? Do I move away with my child and have my relationship end? I do not want to take him out of either of his kids' lives. What do I do?