I read one of his books before i knew who he was, and found although the advice was mostly common sense (if a bit context free) advice, followed by long rants about traditional family's, and backed up by bible. I found myself thinking "what about behaviourism research?", you know there has been progress in the last 100 years. I'm ashamed that the traditional family shit didn't tip me off.

I've found him come up in my google feeds often too, It's insidious.

In terms of convincing your brother about how off this guy is, generally there are how to approach things on line (i.e. you can't always take a logical approach). I've also encountered this kind of thing in my extended family, I've got distant aunts that likely voted for trump, and they are otherwise decent people (i.e. not racist, and supportive).

because he’s otherwise a decent person. I shudder to think what other content he might expose himself to over time and what that will mean for our relationship.

Maybe he's wondering what content you're exposing yourself to, as well? i think it's time you guys talk, but on one condition; To not be easily offended. i'm wondering; would he feel offend by you, or you feel offended by him?