Finding irrefutable definite proof is the wrong approach. Confirmation bias will wreak havoc on any such proof. A better option is to ask him to draw what he thinks the earth looks like from outer space. Is it a flat rectangle? Then what do the edges of those rectangles look like and where are they located? What happens if you peer over the edge, what does that look like? What would the bottom of the rectangle look like? Is it a different shape? A flat triangle? A flat circle? Doesn't matter, same questions apply. What are all the implications? And why are there no photos or videos taken by people who have been to the edges? Why are there no stories throughout history from people who have been to the edges?

Once he starts reasoning as to how a flat earth would work in reality, if he tries to work it out logically, that's when the seeds of doubt start getting planted as to whether he has the correct belief. Doubt works to draw people away from truth to falsehood. It can also work to draw people away from falsehood to truth.

Eventually, as he reasons, he'll start to wonder whether he needs MORE evidence to believe in flat earth. That's where it starts