Help reddit, my son is racist!

I'm a single father working days and weekends as a projectionist at a movie theater. A few weeks ago, my 8-year old son was suspended for saying he didn't like an African American classmate because she was black. Over his suspension, I made him watch Roots. This week, he was suspended again for asking the same black girl if she wanted to play "slaves" with him. I have no idea what I'm doing wrong. I don't have any black friends myself, but I've never condoned racism in any way. Now he is facing expulsion and the situation is causing me to miss work. What can I do?