You'll be best off by basically ignoring the behavior. Drawing attention to it will likely just serve to encourage it.
Your approach of ignoring the behavior is wise; continue with this a while longer (e.g., two weeks). If the behavior continues, place your son in time out when he hits himself. Now, I say this with the assumption that he is developing in a normal way. Is that a correct assumption?
Yes he is developing normally. It just worries me that he thinks its funny, and he hits himself as hard as he can.