Tell him to watch a f'in sunset. Or the eclipse on Aug 21, which will happen exactly to the second predicted by astronomers. Or look through binoculars at a ship sailing away over the ocean. Or look at the stars rotating around the north/south pole at night. Or fly to Australia and measure the mileage between any two cities and ask him which map is more accurate. Make sure to have a return flight through South America and ask him how long the flight took. Or ask him why low tide is always when the moon is at the horizon. Or ask him what causes phases of the moon. Or ask him why the azimuth of Polaris is always equal to the observer's latitude on Earth. Or ask him where satellites are.

Or ask him not be a dumbass.

All of these strategies are incorrect, as they presume that logic, mathematics, and rationality will have any impact on one who clearly places no value on empirical evidence whatsoever.

What you do is put up $10000 of your money against $1000 of his (10 to 1! You can't lose!), on a bet where the geographical mathematics ensures that you are 100% guaranteed to win. Or something--anything--that can demonstrate with real tangible consequences how being intentionally stupid can hurt you and those around you.

Build two identical UAVs, and run a race between two distant cities. One UAV follows a great circle. The other follows a rhumb line. Discounting adverse weather, the great circle route will win, every time.

I second the plane idea. Though actually the seat doesn't matter since he'll want to look out both windows to see the drop in both ways. Else he can just say, "The plane was tilted."

The connection between height and distance that this shows is why old ships used to have a crows nest. I love that connection, and you know how to do the calculation for why it matters. But proving that it is right is harder, particularly if you don't live near a large body of water.

With a bit more trig and a map you should be able to figure out the angle above the horizontal that a mountain somewhere in the visible distance should be. That angle should not be the same as it would be if it was flat. Verify it. Go driving somewhere else, verify it again using the same mountain.

But honestly, I'm personally most convinced by the fact that a phone call to someone in a different time zone makes it easy to verify that you're pointing different directions relative to the Sun.

As much as people like to point to things like eclipses, that relies on knowledge that you cannot verify yourself. I prefer verifications that you can duplicate with direct observation, without relying on outside experts. Because flat Earthers do not trust experts.

Put him on a plane, window seat.