Unfortunately the good stuff is buried under the shit stuff. For every real one there are 100 versions of varying quality that inject flaws and pretend to be the real deal, so when someone investigates they find the flaw and become convinced that the whole thing is bullshit.

The best stuff is usually from the sources that get slandered and censored, but a sheep will always disbelieve it just because the source is "not credible".

So I would suggest starting with things that are no longer conspiracy theories. Look to official sources and point out that at one point this was considered a conspiracy and all the official sources denied it. Literally everyone in authority unanimously denied it. Get him to understand that just because everyone in authority unanimously says something that doesn't make it true.

There are lots of declassified human testing like operation sea spray, syphilis experiments, mk ultra, Tuskegee experiments, radiation exposure experiments, etc.

All of the victims listened to thier doctors. They listened to the news. They believed the science. And because they never trusted themselves to question those authority figures they became victims and often died horribly.