To have this conversation, consider saying: “If people within a group want to claim ownership of a word that has been used to hurt them, that's their call, even if not everyone else in that group agrees with their decision. But if you're not in the group, the word is definitely not yours to use.”
Your son might question this last idea if he is familiar with the concept of an “N-word pass,” by which a black person grants a nonblack person permission to use the term. Here, I'd have you talk with your son about the excellent points raised in a brief video of the author Ta-Nehisi Coates, who notes that some white people bristle at the suggestion that they should abstain from using certain words because they have been conditioned to believe that “everything belongs” to them.
As parents, we want our children to work to understand the experiences of people whose lives differ from their own. Mr. Coates highlights how recognizing who can and cannot use a word has a place in this process. He offers that agreeing to refrain from using the term helpfully gives white people “a little peek into the world of what it means to be black … to walk through the world and watch people doing things that you cannot do.”
Your son won't grasp this all at once. But it's important to start a conversation that can unfold over time about the very uneven distribution of privilege within our society and the demeaning and intensely painful histories behind derogatory words.
Once your son has a better understanding of why hateful language is off limits, you will want to equip him to address his peers who are not there yet — though you can certainly acknowledge that it is often difficult for adults, much less middle school students, to confront offensive speech.
You could say, for example, “If you hear a kid saying something offensive, consider saying to him or her, ‘that term is racist,' or ‘those words are hurtful.' If you see a kid use that language to attack someone face-to-face, you need to tell him or her to stop, reach out to the victim, or alert an adult.”
Whenever an opportunity presents itself, you will also want to model for your son what it looks like to interrupt intolerance. As in all of parenting, our actions teach more than our words.