This is a sticky situation if you try to implement it. At best you temp hide it from the uncle, at worse you double down the ideology because of conspiracy theories and end up hurting your relationship with your uncle, plus Anything you can do locally he could find workarounds for if he wanted to, especially since his friends will know the sites still exist.

He would likely accuse you immediately though as the last person to touch the system is always the one at fault, and you are the one setting it up.

To answer the question though, you could edit the host file to block known propaganda networks(by directing them to invalid ip’s which would make it look like its down) but, that setup is not very effective and unless you can block all of them, hes just going to find ways around it or alternatives, and this system likely wouldn’t survive most current day browsers that are pushing secure DNS such as firefox since cloudflare is going to know how to access it still.

I still don’t think it’s a good idea though, too many things that could go wrong out of it, plus hard pushing an agenda has never been a good way at convincing someone their mentality isn’t right, this will just re-enforce his mentality.