Well, my son does this as well -- more like he's just "testing" rather than hitting out of agression. We've found that saying "no" or even expressing pain just makes him do it more (and think it's funny, as he doesn't understand others' pain at this point). Instead, I've found that showing him how to touch nicely and saying "gentle please" works wonderfully. We've also clapped and given him praise when he is gentle. He loves positive feedback!
At some point, I plan to give him "time outs" for hitting/aggressive behavior. He just seems too young right now -- he thinks I'm playing when I put him in his room and close the door.
At some point, I plan to give him "time outs" for hitting/aggressive behavior. He just seems too young right now -- he thinks I'm playing when I put him in his room and close the door.