Galaxy - forget to mention this in first message, but when you speak to the Head or teacher I really wouldn't mention about the other child being hit. Unless you actually saw it happen then I think that has to be put to one side in this circumstance.
I actually do think that this is worth persuing though. If this teacher has a problem (and yes, some do, espcially in today's school emnvironments sadly) then it needs nipping in the bud and sorting now before any other problems happen.
Oh, and yes - do have a word with your DS to reiterate what he shpuld have done and why what he did do as wrong, and tell the Head that you have done this and that you dont expect him to get no punishment at all, but that is should be appropriate and non physical.