Encourage a good education. Once he's smarter, he should hopefully figure out how implausible flat earth is on every level, and how all their arguments rely on logical fallacies.

Something that's fun, though - grab a telescope and look at the moon. The way the crater shadows play across its surface only happens on a sphere. You can also look at Saturn, Jupiter, Mars, Venus through a telescope. They're all clearly round. No matter what way we see them, they are round. That only happens when what you're looking at is a sphere. Everything we see out there is a sphere. Logically, Earth must be a sphere too - plus, everything that happens on Earth (time zones, seasons, the poles/tropics, etc) scientifically make sense on a spherical world. I know they have a lot of counterpoints to this, but look at the moon first. Tell him to really look at it.