Some people don't realize that it takes one kid to find that out and then the rest of the school knows about it, not even just his class.
I came across this clip again:
View attachment 6213732
It makes me think of three things that could've happened:
Nathan trying to fight back his dad's fame because he is mocked. There could've been older kids who brought it up in school which started this.
Nathan was either directly or indirectly influenced by Tiny's conservative family who kept discussing Stevie's mischief on the Internet. "Not even a real job" is a bit on the nose.
Nathan hates listening to Destiny shouting at strangers all the time so he tries to fight it back to make him stop and pay attention to him instead. Him trying to argue with Steven could be a hint that he wants attention that the random people on the Internet get. He probably sees other kids hang out with their dads while his father is only shouts at people and plays video games.
So either bullying, direct/indirect influence of family, or a plea for attention that's normal at his age.