We believe the things we believe because of how information is presented to us, and the amount of consensus we perceive around us.
The internet has changed those things, so that we experience virtual communities, separate from the actual people in our day to day lives, and shifts our perceptions and realities.
He’s not an idiot for reaching this conclusion, he’s doing something entirely predictably human with the information available.
What might help is to expand the repertoire of material he’s exposed to - take an interest in the reasoning, and let him explain it. You need to teach a topic to someone to really understand it so let him talk to you. There’s a good chance that having to put it in words will help him find any failures of logic, or spot the missing details. Bring more facts and questions to the table to expand the discussion.
An interesting discussion is around the soviets space program, their attempts and the different directions their space exploration took. Their cover ups and secrecy contrast with the public enquiries, scrutiny and criticism of the expenditure (4% of US gdp). The different individuals are interesting too - my point is that you can dig in anywhere and it quickly becomes a very complex topic.
You have to limit the field of information to be certain about anything. That’s what conspiracy theorists do - they create a compelling narrative. But historians, and indeed, all authorities, do this too- the official stories we tell about our world are chosen purposefully. The deeper you delve the more nuance and complexity you will find. Nothing is ever quite simple.
You can fight back with curiosity and information, much more effectively than with arguments and insults. It’s also worth considering that fear, uncertainty and anxiety make simple black and white explanations and answers much more compelling. What’s going on for him that he needs this?