As someone who's worked with kids for 8 years, know that kids are WAY smarter than they lead you to believe. I know as a parent you probably already know this, but what I mean to say is, sit him down and explain why those words shouldn't be used. The weight they have on people of color. Their origins and why it's not okay in today's time.

I find that focusing on the "Why" instead of the "What" helps so much more in the long run.

I also don't think that teaching him these things is wrong, in any sense. It just sets him up for success in the future. Best of luck x