Ah, but according to him... while it's our Earth that is hollow, it's not the Earth in this "space-time". In a different "space-time", the Earth is hollow.
If you're curious, he believes in Falun Gong, of which I've read the first few pages of essentially their "bible" written in 1993 [2]. The first few pages basically says something like... in our past lives we were god-like but we've done bad things so we were demoted to a lower plane, and we kept doing bad things until we've been demoted to the lowest plane: Humans living on Earth. If you become one with Dafa, you can become divine. Two weeks prior, I had read the first ten chapters of the bible (Genesis) and I thought THAT was truly bizarre. But this is on a whole new level! :)
My coworker believes this with all of his heart - I spotted him yelling at the clouds asking for rain so his plants could grow and when it rained 10 seconds later he smiled at me. I'm 90% sure he believes he has the ability to control the weather. <insert "mind blown" gif here>