With any child this age, it is usually the transition between showing how they feel to telling how they feel. You have to access your own child as to where he is in this progression. A child has to be taught how to express how they are feeling. Goodness, many adults don't know how to do this. If he's hitting because he's angry, then he needs to be ok to acknowledge that fact and tell you verbally "I'm angry/mad!" It's ok to be angry, but not to hit is the message he needs to hear. Working with verbal skills will help this along. Also, giving him other ways to show feelings and likes/dislikes, giving him a voice through creative drawing or such might ease this bad behavior.

As with any child he needs to be removed from the situation if he doesn't stop the bad behavior. It may take much longer for him to put the two together, but he will. (Hit, "time out/away") Even if he doesn't learn quickly, it isn't right for anyone to be hit "just because" someone else is having an issue with learning not to. If you don't address it with the other children, they will begin to act out also. (They can't understand something that complicated in it's fullness, to have an adult comprehension.)

Try and see if there is pattern to his behavior. Maybe there is a particular type of situation that overwhelms him more than other times, that really sets him off. Try and prevent that, or lessen it's impact prior to his involvement.

Also, reward good behavior. This takes even more time to be aware and catch him before he acts out, but again, he will learn good response elicits reward.

Sometimes it's the higher intelligence that is causing the behavior, as they expect things to be differently, better, and their frustration level is peaked easily.

(Un)fortunately, it can take another child hitting back that helps the child realize that HURTS!