As far as I know (and if I am wrong, please provide actual FACTS that prove your point), the ONLY evidence of this is that there was a photo taken with Rittenhouse and some Proud Boys AFTER the shootings. Did Rittenhouse know what the Proud Boys stand for or was he a useful idiot? I mean he was 17 years old. I don't know about you, but when I was 17, I wouldn't have known what the Proud Boys were.
Also, the car dealership Rittenhouse claimed he was protecting was minority-owned.
One reason I tend to be cynical about this stuff - people claimed George Zimmerman was a white racist to. But the guy was Hispanic, brown-skinned, and one of his grandparents was black. Turns out he was a registered Democrat who voted for Obama in 2008 and protested against the beating of a homeless black man in 2011. So there is no actual evidence the guy hated black people, in fact, just the opposite.