What do I do about my stepson?

He may not be upset that his parents aren't together because of how young he was when they split... but that doesn't mean he's not upset that his dad is remarried. It takes kids time to get used to such a big change. He is used to having all of his dad's attention and now it is split between him, two step-siblings, and a step-mom. The bottom line is he will continue to act out as long as he can get away with it. His father and mother need to work out proper punishment for him for his behavior and stick to it.