I disagree with all of the "Beat him up back/Sack him/whatever".If you're going to hit him, That would make you just as bad as him. You'd be fighting fire with fire. And thats immature. He hits you. He's immature, You hit him back, that'd make you immature too.
I'm assuming that your cousin can read. Try getting your anger out at him on paper, and send/give him the paper. If he rips it up, he rips it up, but hopefully he reads it. Get him to sit down and talk to you. Have an adult nearby, but not in the room. Make sure the adult can see/hear, but make sure your cousin can't see/hear the adult. Only you would know. Ask him Why he does all this. Is it for attention? Sometimes a younger kid will pick on a relative when they admire them. Maybe he looks up to you and he doesn't want to admit it. When I was 7, I gave one of my older relatives hell because I looked up to them. He has to have a reason for picking on you. Everything has a reason.
I don't really know though, thats just what I think.