I usually respond to endless 'why's with questions that focus on critical thinking. "Do you want french toast or pancakes for breakfast?" "Why?" "Well, which do you think would make your tummy happier?" "Why?" "We eat because we want happy bodies and happy tummies..." and at least with my two, it eventually winds down. Or maybe my questions overload their little minds and they get temporarily choked on all the new ideas.
Sounds to me, though, that his "why"'s are really just "more conversation please" requests. If he answers the french toast vs. pancakes question, then the conversation is over.
And of course, if the conversation starts to delve into the truly odd, feel free to be silly. "Well, pancakes make your tummy want to sing "twinkle twinkle little star" and french toast makes your tummy sing "modern major general". Which song do you think your tummy wants to sing?"