My Stepson Hates Me

My 9-year-old stepson has just recently started acting like he hates me. He constantly asks me why his dad and I had to get married, and he makes little comments like, "If you left, I'd be soooo happy." He wasn't like this before and I don't know what is going on. He has been spending a lot more time with his real mother (who left him when he was 3 so she could party) and every time he comes home from her house, it seems that it gets worse. I don't know if she is putting ideas in his head or what, and I don't know what to do.

I'm not strict or anything. All I ask of him is to keep his grades up in school and not do anything that can hurt him or someone else. I don't think that is too much to ask for. So what could be the problem?