S.S.
I'm guessing that you're in the stepmom role (eow and one day a week, versus the mom role). Assuming that you are, by all means defer to his dad. On everything.
Get me some milk. Go ask your dad.
I don't have any clean clothes. Go ask your dad.
Can I stick this bumble bee up my rear end. GO ask your dad.
I want to go to the movies. Go ask your dad.
I'm hungry. Go ask your dad.
Feel me? He's going to ask his dad anyway.
My stepson is 13 now, and we went through this ALOT - he actually swung at me when I made him take a breathing treatment in the middle of an asthma attack. That day my 8 year old step son, his father and his wonderful mother were informed that he can not stay with me because I can not be held responsible for his safety. Unless and until he showed consistent obedience, he would not leave the house with me unless his father was in attendance, and I could not in good conscience be the adult in charge of keeping him safe (notice the focus on safety, not on making him mind.)
Our family membership to the water park often resulted in me taking his younger sister and my kids to the water park, with him here watching his dad sleep - or aggravating the hell out of him, because my husband worked nights. Fact remains - I couldn't keep him from jumping in the pool and drowning, or running in the street and getting run over, because he didn't listen to me.
He learned to listen real quick. His dad started backing me up consistently, to avoid having to go everywhere with us, and his mom stopped telling him to call me a cunt and that he didn't have to listen to me because she lost her free babysitter.
Hardline - but it works.