I don't really know who to talk about this with so I'm hoping to get some tips to kind of ease my mind. Me and my SO have been together for a year now. We have already talked about marriage and kids and are both divorced. I have no kids but have always wanted them and have already basically raised a little girl for my ex roomate while she worked all the time in my past marriage. When me and my SO first got together his son loved me and even called me mom jokingly and respected me. I had to talk to my boyfriend about this so he knew that it wasn't alright to call me mama or mom but it was also flattering that he loved me enough to even look at me like that. Him and his ex wife still semi talk even though my boyfriend doesn't really say anything to her that doesn't involve talking about or have anything to do with their son. I have talked to her on a more friend level than he has but that's changed due to her disrespecting our relationship and point blank telling my boyfriend that she still loves him and wants a life with him and would leave her boyfriend to be with him again and misses and loves him. She's not the best of parent figures and disciplines him in no way shape or form. He talks back to everyone and has a really bad smart mouth. Anyways my SO told me it's completely fine to stand up for myself against him but I don't want to be that stepmom type figure that's an evil person in any way. His attitude didn't really start until the past like month towards me and I don't know how to address it. I've talked to my boyfriend and told him he needs some sort of disciplining because even he agrees it's because he lives with his mom and his mom spoils him and just lets him say and do and act however and so when he comes over every other weekend here he just does the same thing here. I'm scared he will hate me though if I tell him to respect me and give him consequences for his actions towards me when he acts up towards me even though his dad has verbatim told me I needed to stand up for myself and he wants me to play a stepparent role in his life being as I am gonna be a potential stepmom to him in the future. He is kind of ungrateful and feels like the entire universe revolves around him but he's also an only child and I was the same way so I can understand that part but what I don't understand is the things that come out of his mouth lately towards me. He's come at me with "Why are you so fat?" followed by pinching me and then telling me repeatedly he doesn't have to listen to me I'm not his parent. Then yesterday my boyfriend is getting me a car because I had to give mine to my mom for the time being so he wanted to just buy me one so I wasn't stuck at home and his son saw the saved cash on our dresser and my boyfriend picked it up and joked at me that he was gonna just walk out with a rack of money at dollar general and his son's response was randomly and coldly "if you go out in public with that much money you can get another girlfriend" and I just laughed it off but it really hurt my feelings and my boyfriend looked at him and was like "I already have an amazing girlfriend" and his son looked down at his phone and just scoffed and said "Lies" and he told him to not say that it's not nice and that he needed to apologize to me and to not say rude things like that. 

Thing is though is that this is new from him most definitely and I don't know if it's his mom speaking through him because her boyfriend seems to be failing at being in a relationship with her and she's losing her grip on getting money from him to pay rent at their new trailer and have to move out soon and she's backlashing at me for being with her backburner ex husband as she treats him or if he's just so overwhelmed by everything but I kind of just close off from my boyfriend and his son when he does this. I've gotten better but it's constantly a panic when I know he's coming over now and I don't want it to be like that. I also have an issue with him sleeping with us because he has a bed of his own and he has my boyfriends mom he sleeps with too. We only have a king bed and every time I sleep with them I end up getting kicked out or barely sleep so I resorted to sleeping upstairs or on the couch and when I bring up to my boyfriend that I want him to not sleep with us he says something like well he can't sleep without a fan or it'll be fine I'll scoot him over. He's 7 almost 8 and acts exactly like a baby. He still drinks from sippy cups. I just don't want him growing up being in a baby mindset until he's like 13 because he's babied and acts entitled and disrespectful towards everyone. I know plenty of kids that did that and they had a harder time growing up and making actual friends than anyone else. 

Does anyone have advice on how I should take on this role as an adult girlfriend of his dads that he still needs to respect and listen to ?