Yes .. thanks, I agree with you and think the note of apology for throwing the punch is a really good idea.

My son really didn't do anything much other than to grab the other kid's hood to stop him when running away following the unprovoked punch. Once he had caught up with him, he didn't hit back but simply remonstrated with him. The other kid even confirmed this himself, the verb he was used was "schimpfen".

Thanks for your advice.