Step moms??

My fiancé is a every weekend dad. He split from his ex since his son was 1 and a half. Because she cheated on him. She's married to that guy now. Married him when my fiancé's son was 3. He is now 7 and things are getting out of hand. My fiance and i have been together for almost 4 years now. And we have a 6 month old daughter. My stepson has no dicipline,manners or respect , etc. Not just with me but also with my fiancé and basically anyone. And his mother puts crazy stuff in his head like his step dad is his dad because hes the one that lives with him. But my fiance pays child support and is there for him. He doesnt even call my fiance dad. And i can tell you my fiance is a good father to him. But this woman wants to force her husband as the daddy. So my stepson truly believes now that my fiance was never there for him since he was born. When i got pregnant he started throwing crazy tantrums and his mother said it was our fault because were having a baby BUT he started being this "bitter" ( i dont know how else to describe his attitude change as a child, first time mom) unhappy kid since she got married and  got pregnant herself. He started not wanting me around. Which i understand but i feel like its been too long now for him to come to OUR home and not even want to speak to me without his dad telling him to say hi to me. (Im feeding the baby in the room or in the kitchen and he wont out of his will talk to me) and any bad behavior his mom seems to want to throw all the negative over here and gives my fiance crap over anything she doesnt like. She has full custody so she manipulates things whatevever way is convenient for her. My stepson is not a terrible kid but definitely no manners. I ask him to pick up after himself or take his plate to the sink and he gives me a "why" with attitude..its a struggle every single weekend..he doesnt want to listen to me which i get BUT even to my fiancé. He doesnt want to listen. I told my fiance maybe he needs professional help to help him understand why his life is the way he is. Because he thinks everyone has a stepdad not a step mom though. He asked my fiance if my daughter had a step dad? Like what? To cut it short now lol i recently asked my fiance to have his son stay with his mother at least 1 weekend of the month because im gonna go crazy and lose it. Its that bad. His mom pushes it so much that he comes over here since friday as if she knows and doesnt want to deal with her own son. But my fiance and i got in a fight because he said hes NOT going to do that. I feel so hurt. Because i told him we are always arguing because of his son. He said i knew he had baggage. And i told him YOU knew i didnt so its your responsibility to make sure the story doesnt repeat itself and him having my daughter away from him. He said he loves me and would never want to be away from me or my daughter but is not trying to work with me here. I feel terrible for asking him that but at the same time, not willing to lose my family over a kid tht doesnt want to be part of the family because he cant understand his father has a family just like his mommy. All of this is natural at his mommys house but not over here. Any advice or suggestions? Please no negative comments. Advise yes. Please! I dont know what else to do but i dont want to give up quite yet even though i feel like tossing the towel :(