How do we convince white nationalists he isn't? - well, for a start, white supremacists believe one race is superior to another. That's unreasonable, so I wouldn't bother wasting my time on those losers.
But, most important, it shouldn't be up to us to prove a negative.
The onus lies on all the journos and politicians who day-in, day-out call Trump racist, to provide clear evidence of Trump's racism.
Trump telling Ilhan Omar, AOC, etc. to go home isn't evidence of racism. Those four women and Trump are fighting each other. I believe it was Tleib, the Palestinian-American, who started her political career with 'let's impeach that mother f**ker'.
Trump and those 4 women are having a war of words that's quite ugly. But none of that proves Trump a racist.