OP there is a brilliant talk on racism, and how a non-racist person can make a racist comment just as a non-sexist one can a sexist comment, or a non-honophobic person a homophobic one, It's a talk a friend, who is a teacher, has played to classes to get them talking and thinking, and in my view it's the best mind-opener I've seen on the topic. He talks about well-intentioned people who may say something with connotations they may not have been aware of, and, "It's a conversation which always goes horribly..." as your son's experience shows. "Are you saying I'm racist? I'm a good person, I'm not a racist." He talks about how important it is to have a conversation about "what you said" and not "what you are" and how hard that is, because nobody sane wants to think they've said something racist, because they feel the accusation is that they are racist. He talks about how perhaps it might be better to take that critique - that someone has made a mistake, and said something problematic - instead of their being an inherently bad person whose every thought is problematic. The idea that we're all imperfect and accepting criticism is part of life, and part of changing ourselves for the better.
We live in a world that's racist, sexist, homophobic and snobbish. I catch myself thinking problematic things more often than I like to admit, because I wasn't brought up in a petri dish and I am, as we all are, a product of my culture. The important thing is recognising that it's problematic, and why, and seeking not to inflict that knee-jerk bigotry on anyone else. There's been interesting research done where people are given word pairs to make on screen, and the response time rated, and instinctively people pair black men with more pejorative terms than white. It takes longer for them to pair the other way around when asked to do so, because they are operating against their cultural conditioning. That applies to people who identify as very culturally aware - "PC" as your husband would call it - just as much as those who scoff at the notion there's anything to worry about.
Your son is still a child. He instinctively made a comment because he's a product of his culture, but he also made one without a full awareness of the weight of history behind his words. The girl's parents, and to an extent the girl, don't have that luxury, because they live with the effects of racism every single day of their lives. To your husband, the comment is trivial, in the same way my father-in-law was hugely indignant when disciplined for making a personal and sexually suggestive joke to a colleague when first meeting her. Neither seem to be recognising that the targets will deal with this crap all the time, and that's the context in which the comment is seen, necessarily.
There is a joke that a black woman looks in the mirror in the morning, and sees a black woman. A white woman looks in the mirror in the morning, and sees a woman. And a white man looks in the mirror in the morning, and sees a human being. Like so many jokes, it's funny 'cos it's true. To your son, the comment was trivial. But it wasn't to the target - how could it be? And so it is being logged, not to demonise him, but to assess the prevalence of comments like that. And the fact is, if they didn't maintain a zero tolerance approach to this, then a lot more overt racism would flourish. That doesn't mean your son is a racist person. It just means that he said one thing that was problematic, and maybe he could use this situation as a means to think about what it is like to meet with comments like that all the time, on all fronts... even from boys who are actually nice people, almost all of the time.