Is there any source they wouldn't find "extremely biased?" No, because that's how crackpots work.
Seriously the only way I could actually disprove this, is to build a spaceship, take him into space and fly him around the entire planet so he can see the shape. Then he'd probably claim I drugged him and used hypnosis to plant the memory in his brain instead or something, and that I'm part of the vast conspiracy.
Meanwhile airlines are throwing away millions of dollars by following curved flight plans on a "flat earth." Because we all know corporations love conspiracies more than money!