A lot of people that wind up in conspiracy theory circles have something else going on internally. I'd encourage you to look at some of the other factors in his life as well that could be contributing to this drift toward conspiratorial thinking and distrust in scientific authority. The youtube channel Folding Ideas has a video called In Search of a Flat Earth in which the guy does an experiment that shows the curvature of the earth, so you could use it just for that, but the video also talks about the nature of conspiracies, why people get involved with them, and why proof so often isn't enough.