You could tell him to check out the Kola Well. They drilled 40,000 ft into the earth.

"Much information comes from such holes; for example, the bottom of this hole was about 370°F (190°C).

The structure of the deep Earth is studied today by means that are more indirect. Perhaps the most effective method has been from studying earthquake or seismic waves as they move from one sensing station to another. These natural waves allow us to see inside the Earth as they react to various layers, much as x-rays or MRI’s allow us to view inside the human body." https://www.uu.edu/dept/physics/scienceguys/2003Apr.cfm#:~:text=The%20deepest%20hole%20by%20far,7km%20(about%2023%2C000ft).

So we know for a fact the earth isn't hollow.