Tell the truth, that hearing about these things in such detail gets you down and also that you have other interests of your own. You don't necessarily have to deal with the issue that it's conspiracy theory, just the time factor that you're getting pulled into to something that brings you down and takes you away from things you'd rather be doing. See if that works. It's sorta non-confrontational about the issue of conspiracy theories which I suspect if challenged might lead to a fruitless and frustrating fight.

See if you say you don't believe in the conspiracy then he might try to push harder to prove it and that's the opposite of what you want. And to say it is wacko stuff will just be taken wrong and produce defensive or aggressive behavior that will lock you in even more.

It's an interesting issue, why people get caught up in this stuff. Sure there are conspiracies out there, but most of it is just kinda warped and not really all that healthy. Suppose the conspiracies were all true, what does endlessly going over them do about them? Absolutely nothing. Unless you become an activist for a good cause like environmentalism or civil liberties and devote your time and energy (and money) to such causes then just arm chair quarterbacking about them does absolutely nothing so how could it be productive at all? If it is a true obsession that he can't control then you might have to point that out. It sounds invasive, like you can't get away from it, so even if he were talking about fine wine or types of grass, it's sounds like it has characteristics of an addiction. Enabling that when it's clearly not healthy for you and questionably not healthy at all seems like something to consider in general.