Well lets see here,
1- Can we prove the existence of a God ?
2- Did this God create the Universe and the Planets ?
3- Do we know that " all " the planets in this Universe are lifeless ?
Kids are such curious critters, I would have stammered myself, but replied in regards to whatever doctrine I may practice and have my children
practicing, in your case, Christianity.
I guess when your step-son is older he can find these answers himself, but untill then, teach him however you feel moraly fit.