If he's given you any signs that he doesn't want to discuss it with you, respect his boundaries.
It sounds like the feeling's mutual, that not believing the same thing = crazy. But I doubt it's that intense for all subjects. On-line discussions do not carry any stigma regarding being not respectful of whether the other person wants to be in the conversation because you're not exactly trying to share a house with those people. I've seen it for myself how immersing oneself in a subject on-line, where the most narcissistic people are the loudest, then trying to emerge from that culture to "evangelize" a spouse is disastrous.
It's really hard for the person wanting to put up conversational boundaries to express exactly what the boundaries are on these topics. I can't say, "please don't discuss conspiracy theories with me" without a person in full, on-line evangelism mode taking that as an opening to parse out what qualifies as a "conspiracy theory". Other attempts to categorize the banned topics may go even worse. It would work best if the party seeking to initiate such a conversation be willing to back off respectfully when asked.