Advantage of boyfriend

So I have this issue with my boyfriend being too nice with his family. He lives in a weird situation. His parents are divorced but live together along with his dads gf and their 4 kids. And then there’s him and his brother who are both of their parents. The reason they’re all living together was because his mom suggested they do so “they can help his dad pay the house” which is fine, i understand. But I feel like sometimes they’re all taking advantage of him and his money. First of all, he pays like $600 for rent, and I think (I hope) his dad pays the rest. Then, his car that he already paid off, he doesn’t even get to use because his mom uses it mostly while he uses this old car cause it’s safer. Which is really nice of him, but everytime there’s an issue with the car, like tires or battery, she makes my bf pay for it. Or if he uses the car even for a little bit so we can go out, she’ll ask him to refill the gas tank even though he hardly used it. My bf doesn’t care he doesn’t complain, he’ll do it but sometimes it bothers him he just won’t say anything. Now he’s told her numerous times to save money to buy her own car or whatever, and yeah she’s agreed. She said she would save money cause supposedly she has none. Yet, she’s always going out to casinos and work a lot, plus I don’t think she pays any bills to begin with. She’s alway buying fast food for my bf and his brother so they can eat cause she won’t cook for them since her ex and his girl are out there even though she agreed to live with them. Like it’s everyday she’ll bring home McDonalds, or Panda Express or just junk food for them and it makes me worry that he’s not eating properly. Like his diet is just junk. Then his brother, I can’t stand him. He’s 20 years old, has no job, does nothing but sleep or play video games or watch anime. Yet he’s always asking my bf to buy him food or snacks or games. I just dont like how he’s being treated in that house. He doesn’t complain about it cause sadly he’s used to it. But idk how to bring it up without offending him or having him be mad at me about it. We’re planning on maybe moving in together sometime this year but I worry he’ll back out cause he’ll put his family first. Which is fine, I understand. But if it’s gonna be like that it will bother me to the point where I’ll break up with him. How can I bring up this topic or talk to him about it politely?