He treats his mom the same as he does me, I don't think it's the step parent thing. It's the he likes being a dick thing. He the type of person that he only needs you when he can get something from you, when there is nothing to be gained or nothing for him to use he has no use for you.
Example, this girlfriend he has, he literally has her because she'll do anything he tells her to, follows him around like a puppy etc etc. He broke up with her once because he thought he could get other girls after 4 of them turned him down...he went back to her and she took him right back of course. While we were on vacation he tried to get with this girl on the cruise ship, she shot him down and he acted like a complete ass the rest of the trip.
I could literally tell his girlfriend this and she would do nothing. Yet he tells her that we treat him poorly and she believes it, she literally told our 14 year old daughter that "he's an awesome person and you all treat him like shit". I know this because our 14 year old us that..
This is the type of shit I deal with.