How do I Improve the Behavior of my Brother?

I have a step brother who I visit when I visit my dad who lives 10 minutes away from my moms house (they are divorced) I am seventeen and I can never do homework at my dads house because he always screams and rips my homework apart every time I start doing it. If I lock him out of my room, it will not help anything at all, he just screams and throws tantrums all day. My stepmother and dad have not disciplined him at all and they start to play with him every time he does something bad. I am thinking this is teaching him that behaving bad gets people to play with you and that is a wrong thing to teach. He spits on me, our dad, our grandma, and his mom with no regard to what they think and just laughs and runs away. he also likes to hit anyone and everyone in any way possible if they take something away from him or simply for fun, he will find and throw my dads pair of keys at him and my dad will be too lazy to move and do anything about it and will just scream at him. I am thinking that he has gotten used to people screaming at him and thinks nothing of it anymore. He is pain resistant so when you hit him he just laughs. And if you hit him too hard he cries, but sometimes I have no choice, he just aults me with one of his plastic (yet still hard) toys when I am not looking and bashes my head in and gives me a concussion when I am sitting in my chair doing my homework. I have no idea how to deal with what is going on. no matter how much I talk to my dad and stepmom, they don't do anything at all. I don't know if he has a mental problem or maybe he was just raised wrong but I need to teach him how to behave himself because nobody else will. Please tell me how to correct his behavior in the little time I spend with him or refer me a book that either I or my dad/stepmom can read to find out how to stop this behavior.