Do you think you could show him another option of physical contact rather than a hit? Do you think he is hitting because he wants their attention? or is he hitting because he wants your attention? when he does it, what kind of look is on his face? does he immediately look to you? maybe he just wants attention, and will take whatever he can get?
I would try 2 things:
1) I would give one day where "when he hits" ignore him...if a child comes to you and says "______ hit me", you look at the child (the one who got hit) and say "well, I guess we won't play with him right now!" have that child tell him, "you hit me and I'm not going to play with you now" walk away and play on the other side of the room. I would try my best to not let the "hitter" know I am watching. Have him feel completley ignored. (if he is doing it for attention, this may help him to stop, once you or anyone else gives him no acknowledgement toward his behavior, it may become less "thrilling to him)
2) Would also give a time to "redirect" the situation, IF you choose this option. you would have to pay close attention to ALL he does (not missing a single "hit"). When he hits a child immediately go to him and that child and "make him" do what is right...make him appoligize and then show him another option besides hitting. Have him rub that childs arm or pat the childs back gently, while the "hitter" is demonstrating "what is right to do" explain to him...."this is the way we touch our friends"
Those are the two ways I would attemp to handle it, along with communicating with the parents as well, sometimes children have older siblings who "hit" them....planting the seed! good luck