Indeed, and won't this need to be proven to the prosecution?
My concern is if the lad knows why saying that is wrong, and not just knowing that he shouldn't say it.
There was one black kid in my school I remember once when someone called him "chocolate biscuit". The person who said it obviously meant to cause offense (as it was in the middle of an argument), but I doubt he knew the implications of what he said. They must've been around 11 at the time, and I know both parties reasonably well now, and the guy who said it is by no means a racist (15 years on), and I would say he just didn't know why that name was insulting, and to him it was just like calling him a "twat" etc.
What I think I'm trying to get at is teaching people why racism and racist comments are wrong, and not just telling them that racism is wrong, is required.