Son abused by teacher

If you son suffered injury--sprain, broken bones, etc.--and/or you  incurred medical costs (had the hand x-rayed), then you could sue for the medical costs, for any other expenses (e.g. cab to hospital; lost wages if you took time off from work), and IF the injury was serious, possibly for pain and suffering.

However, if, fortunately, there was no actual injury, just some short-lived pain and anger, there is nothing to sue for. The legal system compensates people for injuries, reimburses them for costs and damages, etc. Without injury or costs, there is nothing to compensate or reimburse. The school disciplinary system, which is seems like you've availed yourself of, is the proper remedy to punish a teacher who acted improperly when there is no injuries to sue for.