If it happened at school the first port of call in to speak to the teacher, especially if you haven't seen the incident with your son (though you did see him hit other children).
Once you've spoken to the teacher (and other parents of children who've been attacked) you need to ask what is to be done about it, and stay in touch for updates, perhaps suggest a meeting on school premises between you, the head, the other parents and the parents of this child. It's what happened to me, and action was very slow at first, but I was like a dog with a bone and wanted the situation resolved, calmly, for the sake of ALL children involved including the boy who was the instigator of it all.
Because the meeting was a controlled enviroment, everyone was on their best behaviour. It didn't turn into a slanging match, the parent of the problem child was not made to feel like a bad parent, and a resolution could be found. The child wasn't demonised either, but a zero tolerance strategy was formed, for ALL children.
The boy in question is now actually lovely, sometimes young children need guidence as to what is and what isn't acceptable and lessons in how to play inclusively. It turns out that this child had felt isolated for a long time, and being a little shit at least got him SOME attention, strategies were put in place to help him.
If you go direct to the parent, especially if you don't know them, you risk it turning into a slanging match, emotions run high on both sides and it's natural for any parent to be defensive.
I certainly advocate involving parents, through every stage of the process, but not in the playground, via text, or with your mates around!! Seen a couple of nasty scraps when that's happened.