Encourage them to learn about it, if this is what they are interested in. There are all sorts of fields one can study in relation to celestial bodies beyond the Earth's atmosphere in all fields of scientific inquiry. If your son likes biology and the universe, astrobiology is the way to go... if he likes chemistry, take the astrochemistry route... and physics, of course, the infamous field of astrophysics, etc. Depending on which field your son shows primary interest, will govern what he may end up studying, if he wishes to delve into this beyond a simple hobby. The universe has in it just about everything any scientist could ever feel the need to study, it's just a matter of finding what interests you the most, to determine where to begin.