If your kid doesn't fight back, the cousin will continue hitting him. I learned quickly when I was a little guy to slug them back or be prepared to take a lot more abuse. Nobody "official" is going to tell you that so let's just keep it a secret between us and your son!

Before you encourage all out war, how about teaching your son to set limits verbally. He can tell his cousin to keep his hands to himself. He can refuse to play with the cousin as long as the cousin insists on being rough. He can tell you or another adult (and you can speak to the child's parents and ask for their cooperation). You yourself can separate the boys and punish the offender. If necessary, never leave them unsupervised.  If absolutely necessary, let the child's parents know that the two boys just can't play together at all until this changes (or they grow up a little more). In other words, you have lots of options other than teaching your kid to be as ugly as the other one. All the best,  Sarah Chana

I think it's up to the adults to make sure that violence is punished. Your kid shouldn't have to defend himself.