The anti-intellectualism, conspiracy prone tendency, and dependence upon self-published stuff that can’t make real arguments suggests to me that he’s not a very smart guy, and I’ll bet, deep down, he knows that. And that can be true even if he’s wildly intelligent in one area.
My guess is that he wants to feel in control and in the know, and masks his fear that he’s not with crap like this. It’s been bubbling for a while, but it got nastier because of his divorce, so he’s reaching for anything that makes it not his fault permanently.
No fact, argument, or intervention is going to hit home, both because this is emotion based and because it’s been going on for years. If you walk out of a good session with him and think you’ve made a dent, he’ll soothe his angry and confused mind by falling right back into his unhealthy habits, because they make him feel better.
The most you can do isn’t to oppose, but to provide alternatives. He’s listening to these things in his car and getting mad all the time? Find some audiobooks on safe topics he likes and let him borrow them, and ask him follow up questions to engage him. He always reads some drivel right when he gets home from work? Tell him that you’ve started doing a fitness challenge and you want to see which of the two of you can walk more steps in a month. If he brings up a topic that’s going to be a problem, sigh or make a face and change the subject; ask about his kids. Talk up therapy and introduce it as a normal, healthy thing that has helped you or others out. The goal is to help him find peace and happiness and connection outside of his obsessions so he starts to rely on them less until he no longer needs to hate others in order to be happy as he is.
The biggest issue is commitment and time. It can take far, far more effort to deprogram someone than for they to get hooked, and ideally it’d be a family or community aid sort of thing, because it’s a huge undertaking for one person.