Ah, welcome to the world of the outsider. Been there, done that and am still doing it!

I've learned that this is something your husband has to take care of because if you try to force it, you will still and again, come out as the bad guy. It's a no win situation for you. If you agree to everything, right or wrong, you are allowed to be the good stepmom but when you say no, sorry, you can't, you automatically go into the evil stepmonster mode. It's the old addage, blood is thicker than water.

Maybe if you and husband can agree that the rule stands, no more borrowing, etc. and the Stepson understands that it's both of you and not just you, it will sink in.

Another thought is, your husband may have assembled this monster so he has to unassemble him. It may take a while because kids are spoiled for a reason...they were allowed to be that way, so it could take a good, long time to undo all of this.

Just know that I understand how you feel and detachment works best for me.