Tangential question: how do you get around how overtly racist these bands are? You're citing Darkthrone, for instance, which is basically the poster-child band for teenagers using metal as an excuse to shoot people and burn things.
No, they aren't. You're thinking of Varg Vikernes of Burzum, and maybe Faust from (early) Emperor. Fenriz, Nocturno Culto and most of the rest of the Norwegian scene had nothing to do with Varg's 'black metal politics'.
As for getting around how "overtly racist" those bands are: you can't, at least with releases like Fullmoon's United Aryan Evil. If the subject matter bothers you, you simply don't listen.
I can appreciate a lot of metal, but do not generally enjoy black metal just on musical grounds. But I find with repeated exposure I can usually find something to appreciate and enjoy in almost any music. When I was a teenager I had friends who were into the Black Metal scene (mostly because of theatrical bullshit like Vikernes and Euronymous), but after learning how racist the bands were, I decided not to acquire a taste for the music.
Varg has maintained that Euronymous was planning to kill him. When asked why he thinks Darkthrone allowed him to write some lyrics despite being a pariah in the black metal community, he said that he thinks Fenriz believes him about Eronymous' plans. He said Fenriz may think it was unfair that Varg was charged with murder for killing a man who was planning to murder him.
That's obviously not how the law works, and what he did was deeply wrong. That said, I can kinda understand feeling some sympathy for someone in that situation.
Who knows if Varg is right about Fenriz. Frankly, Varg believes some pretty crazy stuff, and I'm not inclined to trust him about much of anything. But you never know. Maybe Fenriz took pity on a really screwed up kid who made a terrible mistake.
Anyway, onto the subject of racism. I think there was definitely a bit of racism in the early black metal scene. It's hard to know if it was genuine or just some kids trying to find a way to shock people. Either way, it seems like that's mostly gone these days (I can think of a few exceptions, but they're seemingly rare).
Black metal (and the people who create and listen to it) has grown up a lot over the last couple of decades. Fenriz even got elected to local office in his hometown!
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.
[0] http://www.ranker.com/list/best-black-metal-bands-and-artist...
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:
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.