Your husband is the only person that can resolve this and if he respects you/your marriage, he will. He needs to enforce your existence and authority daily. Remember all those movies/shows where the kid asks the father for something because they think he will say yes but SURPRISE- the father says, "go ask your mother." \*laugh track\*

Your stepson doesn't see his dad and you as a cohesive partnership. You're not his mother but his father and you are a unit in life now and he needs to witness that. He needs to SEE his father agree with you, run things by you, etc. 

Until your husband shows his offspring what's up- you're S.O.L. 

It's a red flag against your husband that he has such a bratty mcbratwad and he hasn't stepped up. Sometimes the roll of the dice plays against you and you end up with a fiesty little seedling, but that only means the parental unit has to step up and work that much harder at teaching them how to navigate reality/people/etc.