K.O.
actually, my only advice is to find out why he is hitting. There are many reasons to why children go threw this stage.
One my be that they are frustrated at what the other child has done to them, like taking a toy he wanted or so forth. If that is the situation you need to talk to him and explain that he needs to use his words and encourage him to do so. Like: Does that make you mad, sad, angry. Then give him things to do or say when he gets mad, sad, or angry. Such as telling the other child, telling an adult, or redirecting the child to something else.
Now, it could also mean that your child just might like to hit. He doesn't realize that it is hurting the other child. It is just another form of playing around. In that case you need to explain to him that it hurts his friends when he does that. Then give him examples of when he got hurt... like: remember when you fell and you cried? You were sad when you got hurt. Now, it hurts your friends when you hit them. If you want to play with them go get your truck you brought today and play together.
I know this may sound too simple but it truly works. Kids need to be able to recognize thier feelings and the feelings of their friends. When they do recognize they are sad or they are happy, then they start to feel the emotions that their friends are feeling and can tell you : Jimmy is happy as he laughs with him.
In the last case it could be that your son may strive for some attention. If the child feels like someone is getting attention more than he is, he might act out in order to get attention. Even if that attention is bad.
So, if that is the case, make sure your child is getting some one on one with the adult in the room. Not for a long time, just about five minutes here and there. That makes a big difference. You usually see this in a older child who doesn't have siblings and they go to a day care or if the older child has a younger sibling.
Hope I helped you out.