S.S.
I have 3 kids and this is pretty common and he will keep doing it until you make him stop. Who knows why he does it....who knows why kids do half of the things that they do. My suggestion would be everytime he hits you immediately pick him up and put him in time out or his room whichever works for you. Don't tell him to stop, don't argue with him about it just pick him up and go!! Hopefully he will catch on and knock it off!!