Your comment makes me wonder if a large part of what's driving this is the proliferation of so many disparate (sub)cultures, and specifically the brain drain away from the subcultures that had been the traditional societal defaults.
When you were at school, you were both part of the same social hierarchy. On the topic of math proficiency (general smartness, really), it was easy for him to perform a mental check and see that you were above him in that aspect (even if he was above you in other aspects), and so he should listen to you.
But then you probably went your own way, essentially getting out of his way with him continuing to advance in that traditional subculture. So now he feels he's earned being considered closer to the top of the hierarchy, even though it's no longer a pan-society hierarchy!
And then he also cannot understand where to slot you into his mental model. So you just get written off as an outside attacking force to be opposed.
This also explains why they continue calling themselves "conservatives" despite gleefully working to destroy our institutions and standing in the world - what they want "conserved" is their perceived prominence of the subcultures they bought into.
That's an interesting point of view: in a way he "doesn't know me anymore", so why would he trust me? And he has to trust me because if he doesn't understand why I am better in this specific topic (maths), then the only thing left is his belief...
I once heard flat-earthers in a train. One was saying "It's like school, e.g. physics: they purposely make it such that we don't understand it to confuse us. Because we're not worse than anybody else, so we should understand, right? If nobody understands, that's because they bullshit us!". I really wanted to say "counter-example: I could understand physics at school :-)".
When you were at school, you were both part of the same social hierarchy. On the topic of math proficiency (general smartness, really), it was easy for him to perform a mental check and see that you were above him in that aspect (even if he was above you in other aspects), and so he should listen to you.
But then you probably went your own way, essentially getting out of his way with him continuing to advance in that traditional subculture. So now he feels he's earned being considered closer to the top of the hierarchy, even though it's no longer a pan-society hierarchy!
And then he also cannot understand where to slot you into his mental model. So you just get written off as an outside attacking force to be opposed.
This also explains why they continue calling themselves "conservatives" despite gleefully working to destroy our institutions and standing in the world - what they want "conserved" is their perceived prominence of the subcultures they bought into.