Great that he wants to talk to you about this!
Anyway I think I would first turn the question around to him. What does he think or know? then go from there. That way you can see how much he knows and how accurate the information is try to correct the wrong information and provide information on the level he is at.
My DD is only 6 and has no interest in knowing at all (but we do give her some facts and info). But my DS, 4 ask questions, granted very simple ones but I have a feeling he will be wanting to know sooner than she.