he could have seen this at his dads house. or the girls could have gotten under his skin and he couldnt take it anymore.
Does his father know about these incidents? If not, I think you need to tell him. If he's letting your son watch violent cartoons/tv, or if there is something going on in his home that emulates violence, you need to know.
I wouldn't be so concerned if he hit girl #1 in the cheek, and there was no bleeding and it was a one-time thing for him. But he ended up hitting another girl hard enough to make her bleed in the same week. Kids with good social behavior and very limited aggression don't just up and do that. Somehow the idea got in his head and he put it into action.
Monitor the movies/tv he's watching, and talk with his teacher about how she thinks he's being treated by the other students. And talk to his father.