Do you have some references for this (I was afraid this was a problem but hadn't looked into it)? Been wanting to talk to my son about that. He loves the music, he's seemingly not racist, but repeated exposure to that crap is not good.
Other than that, Varg (of Burzum) is also a neo-nazi, but he hasn't written any black metal in 20 years. In fact, he stopped writing black metal around the time he became a neo-nazi.
Aside from that, I don't think you're going to find much popular modern black metal that is openly racist. Maybe some band members are secretly racist, but they know better than to talk about it if they are.
My (now) wife introduced my to black metal over 15 years ago when I was still a kid. I turned out fine, and still have no tolerance for racism.
If you look back at the music you enjoyed in your youth, I'm guessing you could find some music you like made by people with a questionable moral compass. However, you don't have to think about such things with music, sometimes it's just about the way it sounds.
If you're concerned about your son, why not see if you can broaden his musical horizons outside of metal, maybe there's something you'll find that you'll both enjoy. If you'd like a starting point, perhaps your son may like some classical composers like Bela Bartok, some of his pieces are easy for metal fans to appreciate:
[0] http://www.ranker.com/list/best-black-metal-bands-and-artist...
I was really into Darkthrone and still think they're okay, and I have nothing against any race or ethnicity.
Just one that I am aware of, and they apologized later, was their comment about "Jewish behavior".
Also, this:
Den Norrøne Rase må Slakte den andre
når blåmenn dunker for tungt på vår dør
Being concerned over it sounds like wanting to ban violent videogames, overprotective. Like hardcore Christian parents against their children listening to rock music, but maybe not as much since black metal is basically how those kinds of hardcore Christians see normal rock music.