I had a similar situation with my step daughter a few years ago. I believe her behaviour stemmed from the half and half lifestyle she lead. Whilst she got used to it I am sure, she was always on the move, two bedrooms, four sets of grandparents, different houses after school on different days. Her Step Father, stepped back and kind of ignored her, I presume because the tantrums and generally bad behaviour got the better of him. Looking back, I think that made her insecure, and she would come to us all guns blazing, with behaviour her mother couldn't control. My DH wanted her to feel loved so was pretty inconsistent with his discipline, so it was left to me a lot of the time and it was difficult.
I was a step daughter myself, so I know what it feels like. My step father HATED me, I was indeed a cowbag, but he was the adult so I think he should have tried harder. I love my Step father with all my heart now and he loves me, so there's a happy ending for you there. However, being a step daughter myself and the experience of being hated, made me really try with step daughter, but she was a really horrid child. Anyway, I am rambling. My point is, maybe it is the whole situation, the upheaval, the step father, the broken routines, the lack of discipline for many years now that have made him what he is. You do not have to like him, and it sounds like you try really hard and are the adult and are kind to him. All you can do is your best. My Stepfather did not do his best and it all turned out fine and so did I. All children are different, but I would suggest that you can do little to change your step son now. Just do the best you can by him and the chances are he will look back and appreciate it, and he probably will turn out ok. At the end of the day, bad behaviour or no, he is loved and that is what counts.