hi there- I dont know how much advice, but Ive been teaching and working with this age children for a few years and its quite commen , so dont worry.
What I would do is at the beg I would explain it was wrong, and then every time he hit, I would make him sit down by the wall (not corrner, never liked that theory)...some people call this "time out", and for 5 min he wouldnt be able to play. I didnt even say anything to them.I would make a big fuss over the child he hit and totally pay no attention to him at all, just take him by the hand firmly and put him down, and I did this every time he used to lash out at anyone. , No matter how small the incident was. Sometimes the child likes the attention he gets by acting like this, maybe he doesnt get enough positive attention, so I wouldnt constently praise and reward him for every good thing he did and make a big deal from it, so he would see when he hits children, nothing comes out of it but when hes good, hes staright away recognized, and prasied AND gets attention!!!..not saying this will deff work but I belive strongly in this methos, maybe talk to his teachers and tell them this, g'luck