That is the best advice I have for you.  If he sees you treating all people with kindness and dignity regardless of their race, religion or sexual identity it will register.  I agree with Cajun mom, do all you can to expose him to different people, cultures and belief systems so that he experiences them as people, not "others".

I grew up with a horribly racist, sexist, antisemetic, if you ain't my shade of pale and flavor of Catholic you aren't my equal kind of guy as a father. This was in the 60's, and my mom, for many reasons, did not divorce him.  She did correct his language and behavior in the home, exposed us to as many cultures and religious festivals as were available in our rural area.  She also encouraged us to read on any topic, but encouraged geography & the classics.  I think it balanced some of the awfulness my dad spewed.  
And while racists come in all stripes so to speak for many reasons they have found a home in the conservative movement and element of the Republican Party.  I don't say this to inflame or uoset anyone.  It is just a sad statement of fact.  I think recent events present a wonderful opportunity to discuss how we treat others in general and how to safely discuss different points of view without screaming at each other.

I'll leave you with this-- once upon a time being pc was considered being civil and polite, addressing people by the terms they prefer and with kindness.  And a snowflake was a wonderful individual work of art because of its unique qualities.  Being pc and a snowflake are nothing to be embarrassed about.

This is exactly why I focus on behaviors rather than words. Actions, are the greatest tell of character than any words will ever be.

Like  you, DW and I are registered voters but do not belong to any political party.  

Focus on behavior rather than Race, or any other cultural, gender, or sexual orientation demographic.I couldn't care less about someone's background. I care about their behavior,  performance, and their qualifications.  Things they are in control of.

I landed on a very powerful perspective recently that I firmly believe in and support.  As our culture is in upheaval and freedoms and rights are being assaulted seemingly wholesale.  

Prosecute the crime. Do not prosecute the freedom.

Hold the individual accountable. Do not demonize an entire demographic.

My son is one who can fully accept this perspective.  He is extremely conservative, gay, and the eldest sibling to three younger SpermIdiot spawned half sibs. The youngest two are biracial boys.   He holds himself accountable for his performance, he holds his siblings accountable for theirs, and does not either deify or demonize any group or class of people.

I am proud to follow his example.

You cant do anything about it. It may change when they grow up and realise its not cool or ok

My SD19/SS18s BM1 is the worst. When they were younger she would say things like "i dont want my kids around no fkin indians" because we would take them to multicultural events....(everyone brown = indian in her head)

SS15/SS12's BM2 is also prejudiced and told her children to tell me "im sorry but I am not like you, i am xyz". I was a bit puzzled because I never tried to make them conform to me or where i am from ans we never talk about these things haha. The funny thing is that she is half mexican but denies it and hates anything to do with it.

All in all you cant change them and you should let them do themselves.

I think you should definitely talk to SS13 therapist about this problem. Only a specialist can find the best approach to such delicate issues. You can try talking about it at home in a very subtle way. I mean, it doesn't have to look like a lesson. Usually, children are imitating their parent's behavior, so try to be a good example for him. Unfortunately, nowadays, racism is everywhere, and it's quite hard to deal with it whereas those who fight for equality are easily being shut up https://calgaryherald.com/news/politics/ucp-candidate-caylan-ford-resign.... Try to find more multicultural activities for him, I'm sure he'll get used to this environment and won't judge people by their skin color.

Not everything do people or kids need therapists for, especially because they hold their own views and you aren't sure if they will allign with yours.  I would just lead by example and have conversations on race and differences.  There are a lot of amazing films (movies and documentaries) on race issues.  Make it a date, one evening of visitation you watch a film and discuss it as a family.