Dealing with agression can be very hard. When did the agression start? You want to look at what may be triggering these actions, did he see others, etc. When he justs mad and hits that's because he doesn't know how to express his feelings. When he's getting mad stop him, hold him tight, and tell him he's getting angry. Explain to him that he shouldn't hit people and if he needs to to hit a pillow. I know it sounds werid but that was some advice from BabyCenter.com Go to the site and look around. There maybe other forms of advice.
If he's hitting when he's frustrated, try to limit those events that trigger the hitting response.
After he has hit another child or person, don't give him all the attention. First talk to the child/person he hit and aplogize, but also make him aplogize even if your the one that has to say it. Then let him know that it was inappriporate behavior and he'll be having a time out. Aviod using the words "no" and "bad". He may be doing the behavior for attention and if he learns it wont get him what he wants he should stop.
Also remember to praise him when he doesn't hit. We often overlook when they do things the proper way. If he gets mad and walks away or goes to hit and doesn't, thank him and tell him that was great.
I hope some of these things can help.
-S.