My step son is driving us nuts!!!

He's going on 3 next month. He gose back and forth between us and his mom.. We usually get him for a week or two then he gose back for a week maybe less.. So lately he's been a HUGE cry baby, crying over the simplest thing like telling him to sit down when he's jumping on the couch, when he doesn't get to watch the show he wants because we try to make it fair for the other kids that are here. Lately hes been hitting my niece, who is 3, I've put him in time out a few times a day for it, nothing seems to click with him he just keeps hitting her and not listening. I dont believe in hitting children.. I dont know what to do, I yell, I tell him no and I know he understands because he never use to be like this. I've had no choice but to put him in a separate room, and every time I do, he screams and screams and pounds, kicks, until we give in... I dont know what to do anymore, I dont know if this behavior is coming from his mom, I wouldn't doubt it honestly.. Can any of the moms give me any sort of advice?.. I'm going crazy over here :(