Mom who needs advise

I'm at a loss here. My 3 year old son continues to hit, kick, punch, head butt and tries to wrestle. I've tried to teach him when its appropriate and when it's not but its absolutely continuous! And to top it off I'm pregnant. It's starting to worry me, his behavior. He doesnt do it out of anger, hes playing and always laughing and smiling but he doesnt understand it's not always appropriate. I've tried everything and so has his daycare. But to make it worse he head butts and punches my stomach as hard as he can. Even after explaining it could hurt the baby and theres a baby in there. I'm at my wits end.