That's the other thing-if there are other kids involved, especially if there are eventually kids in your relationship as well as your step son, they must all be treated the same. I have a friend who struggles because the dad is a walkover, and his girls run rampant. The mother is more firm, and her kids are well behaved.
Trouble is that the dad won't allow the mother to discipline his kids, despite them literally tearing the house up. They've been together for ten years-I don't know how they have lasted.