I quit

I really don't understand why... I sit here and i'm upset, no to be honest i'm actually mad. I have 3 kids and out of the 3 two are biologically mine and the other one is my stepson. I try to love him, and i try to treat him the same as my girls. I do things for him, i buy him things, i take care of him, i tell him i love him and that i would do anything, but yet, he's sooo disrespectful, and just flat out does not care at all. I don't understand. He's almost 8 years old and he still pees in the bed, and he still pees on himself and poops on himself while he's awake and he doesn't care. I don't get it. What am i to do? I just don't know. I can't handle it anymore. I have told my husband that i won't do it. I know it kills my husband, but what else is there to do. I pray and pray for guidance and i get be patient, so i do, but it does nothing. He chokes my daughter, he hits her, and she cries and tells, and he doesn't care. I'm open to suggestions. I have done time out, i have done taking things away, i have even spanked, but nothing works with him. I have talked to him, his father has talked to him and nothing. When i asked him do you do this at your moms house he tells me yes. The bad part is its rubbing off on my girls, and i can't deal with it. I didn't ever expect for my life to be this way, especially when it came to kids. I can honestly say this is why i NEVER wanted to marry someone with kids. My kids are too young to understand whats really going on. However, Chey is getting to the age where she is learning and knows right from wrong, but she doesn't get why her brother gets away with it when he's with his mom and she doesn't. I try to explain to her that i'm not his mom and i expect more from her, but she doesn't get it and so i don't really know how to explain it to her. I just wish i could figure it out myself.