I like the answer by D. H. However re-reading the question and the comment by C. W. it seems possible that yours boy got it right in a way.

Indeed, the people below equator are tilted or upside down with respect to us. This still can be explained as suggested, of course we know.

But the two situations are different. 

It comes to me to realize that a sphere and Newton law are not enough to explain why somebody in Australia is not walking on its head. So the fact that *we sense up and down* plays a crucial role. 

Unfortunately it is a mix of physiological psychological factors, I guess. 

You should make sure what your son in asking, in other words. 

I would say that gravity is at center of Earth. The latter being spherical, gravity is *under our feet*.