This is my first post so hope i get it right.
My son is 6 and I was informed by his teacher (some time after the event) that he had made a derogatory comment about a Chinese boy in his class - specifically to do with the shape of his eyes. I was mortified.
However, it threw up a couple of points: the school didn't think it necessary to call me in at the time of the incident and simply referred to it as "immature silliness"; my son had no idea that it was rude- he was bewildered when I explained it to him - he was just repeating something another boy had been saying and didn't have a clue that it would cause offense.
I can totally understand how upsetting this type of comment can be to the recipient and I think it is vital to use the opportunity to explain racial differences and how we should treat each other, but I disagree that this makes my son a racist. 50% of the kids in his class are from different ethnic backgrounds to his own and he sees this at totally normal so I feel he is well used to seeing kids that appear 'different' and doesn't even notice it, unless it is pointed out as in the situation I have just described.
I believe that its very difficult to find young children that are genuinely racist; even kids who are exposed to abhorrent racist behavior/language from their ignorant parents who then appear to be racist themselves are only repeating bad behavior and don't really have any understanding of racism. My son might have made a very silly and ill-informed comment about this boy, but it could just as well have been a silly and ill-informed comment about another kid and could have been related to anything. He's a child and as much as we hate it when our kids cock-up, it does happen from time to time! Perhaps its the same in your situation?
My view is that I constantly try to teach him compassion and tolerance and kindness, and most of the time he behaves that way, but he is 6, he is influenced by the 'cool' kids in his class and sometimes he makes mistakes, like all of us.
In your situation, try to establish if this kid has made other 'racist' or offensive comments, reassure your son that what he said was just rubbish and rather silly, and if you genuinely think its racist contact the school, otherwise perhaps you could just leave it at that? As long as your son understands what was said isn't true and that he should rise above it, perhaps that is more important?