He's probably feeling disadvantaged, is inadequate in some way, probably depressed and has latched onto someone or something who seems to have answers.
YouTube, 4chan, Reddit and some flat earther forums, more than likely.
Conspiracy theories all seem to stem from the same assertion that there's a shadowy 'élite' (Jews, alien lizards, bankers, Royal family, Freemasons, Illuminati...take your pick) controlling world events to keep us in our place and enrich themselves.
If that is true, then that explains why people don't get anywhere in life and there's no point trying to better yourself as the Man/the Establishment/the alien lizards or whoever will not tolerate it.
The fact that it's damn near impossible to keep any kind of secret these days, there are plenty of people who are making successes of their lives with hard work and that any such conspiracy would involve hundreds of thousands, if not millions of people means nothing.
The conspiracy theorists has answers (nonsensical) for any explanation.
Usually, any argument ends up with
"Yeah, but how do you know though? How do you know? Are you an international financier?"
Of course you're not
"Well, how do you know?"
Well how do you?
"I'm educated. I have been educated!"
It's tedious.
The comments thing is probably more of the same.
His self esteem is so low, he's constantly on the look out for anyone he can compare himself favourably with.
Fatter, scruffier, more stupid etc.
If there's no one else, he invents scenarios about them
"She's probably....."
"I bet he's......"
Try to limit contact as much as possible.