OK, I am a step-dad to two girls, so i speak from experience.
First of all, all children will play one parent off against the other - biological, step-parent, who cares - and trhe more complex the situation, and the more background grief, the more they exploit it.
You have to put the following rules in place -
1. You and your husband will back each other totally in regard to discipline - even if you think the other is wrong, you sort it out in private, but to the children, you are united, and cannot be divided (sounds like a song doesn't it ....)
2. Your husband needs to instil respect for you in his children. You are his parter nof choice, and they must respect you, and he must enforce that - and the same for you if your lad starts testing the boundaries - because that is what this behaviour is.
3. Get your husband to tell his ex-wife that none of you will fight through the children. Anything that the children bring to either household will be discussed calmly and in an adult fashion - not ading fuel to the flames by believing one child against one adult - that simply leads to endless pointless rows.
4. Start a regime with all the children -
I love you very much, and I love it when we are nice to each other, it makes me very happy. When you make me unhappy, I will make you unhappy, and continue until you are nice to me again, and then we can go back to having great times, and enjoying being together.