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.
You'll be best off by basically ignoring the behavior. Drawing attention to it will likely just serve to encourage it.