Now it's White Nationalism.
This can take two quite different forms in someone's mind.
1) Does he believe white people are superior to non-whites (more evolved - he might not put it that way, might not even believe that, but that's where this idea fundamentally comes from), and therfore should rule over and be privileged above the non-white people?
OR
2) Does he believe white people are a different nation, and his local area (town, state, entire USA) should belong to the whites, and non-white people are just as entitled to their own land but should go back to Africa or wherever as that's their place (and all the Jews should go to Israel).
In other words, is he a white supremacist, or just a white nationalist? Those two are quite different and should not be confused - one says "blacks should live here but whites should rule them", while the other says "all blacks should leave" - they're almost complete opposites when you think about it. Once you properly understand which you are dealing with you can have more intelligent conversations about it.
But you're not going to persuade him yourself with your own reasoning. He has to persuade himself. Just ask him lots of questions - getting him to ponder this might help you to steer his mind into better directions.
Assuming he's on the spectrum, he should enjoy working this out and explaining his views.
If he's a white supremacist - then he may harbour no animosity towards blacks, might even see valuable features in them (consider them physically stronger for instance, more suited for physical labour) - but may just think that whites are smarter and should rule them, in the old master / slave relationship. He'd like to have more blacks around, provided they do what he says...
If that is the case, ask him exactly what people group should be supreme, and for what measurable reason.
Ultimately find out whether the Asians should rule over the whites - in measures of either academic achievement or brain development, it is Asians that top the charts, not whites.
What about the Jews - are they a ruling or subservient class? Where do they fit in the hierarchy - above the whites or below them? Why?
With that line of reasoning - conducted by himself in response to questions, not suggested by you - he might decide this idea of supremacy isn't such a good idea and might give it up of his own accord, since he wouldn't actually be top of the pecking order!
On the other hand, if he's a white nationalist:
- What are the racial divisions he sees?
- Which land is assigned to each race?
- Why is that land assigned to that race, and not another race? For instance, why should the USA be assigned to the whites and not the native American Indians? Do the blacks get all of Africa - so do the Afrikaners have to leave South Africa in his ideal system? Do the Jews get Israel - and must they be deported from the USA and be sent there?
- How much area does each race get - is it based on population?
In other words, can he explain his own position using rational logic - or does it fall to pieces when he tries to do so?