How to solve this situation? any help please!!!!?

My step-son ( 10yrs) is driving me nuts he is about to leave in a week after spending 2 months with us. But he has beign very nice specially with me. I can't say anything god or bad because he will not obey me at all.For example my kids know that no food or drinks are allowed in the car or in theri rooms unless is popcorn or drinks are in zippy cup. He is always taking drinks, candies & food to the van & I always remaind him but he ignores me I have got to the point that I have to remove what he haves because I believe it is not fair, for my kids & like the 3 to have the same dicipline. I have told my hubby but he says is because I just like to argue with the child but it is not true. I just warned him if he is not going to support me on trating our kids the same way then the problem will always persit & or relationship will en up very bad. Th kid says his mom told him only to obey his dad & that is what he is doing. But I believe is not like this! What 2 do? Advices please!! :)