P.M.
I wish we could have seen the behavior, because it could be anything from aggressive anger to coy teasing – I recall many of the boys in 2nd through 6th grade showing their interest in girls by pushing, poking, pulling, yanking hair, hiding books, and teasing in their coltish, clueless way. And many of the girls took this as a mark of interest from the boys, and giggled about it with girlfriends.
Because you don't say otherwise, I'll assume the girls were actually frightened or hurt, and not just razzing and provoking your son to get a reaction, and that your son is not suffering from A.D.H.D. or other disorder that would give him less-than-normal impulse control.
I would give him a consequence that fits the behavior. I'd have a chat with him about what happened, completely reserving my own judgements for the time so I can find out where he is on this. (Usually people, including kids, are reluctant to express regret if they are shamed or scolded into a corner – they'll just go on excusing their behavior and trying to explain why it's not their fault.) His side of the story could be informative and make a difference in the "sentence."
Which might be, for example, to have him write a note to each of the girls, if he seems sorry that he got too rough. Struggling over notes would be a most corrective and restorative outcome for a nine-year-old, don't you think? It would help to heal any lingering hurt or anger in the little girls and their parents, and be most instructive to your son. You can make a mistake, and make it right, and move forward with life and your friendships.
ADDED: I'm totally in agreement with Victoria, that boys need instruction, early and often, about treating girls gently and respectfully. And I've still found it a useful technique to talk to a kid without judgement when I want to find out what their current thinking is on the issue of the moment. Few kids will be as forthcoming when they are flinching from a lecture or anticipating punishment.