You could go to www.straightdope.com and click on the “Buy Stuff” link. There, you’ll find a book called “Know it All”, by our own Ed Zotti. Here, I’ll save you time: https://securesite.chireader.com/cgi-bin/dopestore/buystuff.bat#kia
Click that. You can also pick it up at a B&N, or your library. Buying it would be best though. That way, your kid would have reference material on hand all the time. Plus, it’s just a good read.
Don’t worry, we’re all ignorant about some things. Just don’t ever bullshit him about something you don’t know - sorry, can’t remember the earlier poster who suggested “Let’s go to the library and find out together”, but he was right on the money: Dorling Kindersley {DK} put out some wonderful books that I wish I’d had when I was a kid - everything from Skeletons to French Impressionists: large, beautifully illustrated, and not dumbed down in the slightest. God, I learn from them.
Lowercase is two and a half, and LOVES going to the library. I’m so proud. And the museum - well, my son in a museum is a whole separate thread. Suffice it to say that he thinks they ought to be open 24 hr.
Show him how to ask on the net, the peek function is cool watching other kid’s questions go through.
Another vote for don’t bullshit him - I remember once being asked by a kid who was perhaps a couple of years older than him, on being fascinated by our log fire, “why is fire hot?”. I ended up explaining all about atoms. And he took it all in.
Bullshit was never an option. 
Another book you might enjoy to give yourself a grounding in order to stay ahead of an endlessly curious kid: An Incomplete Education, which has some flaws but which still provides a quick baseline across a whole swath of disciplines, from science to art and architecture to history and more. Presented in bite-sized pieces and easy to read. Everything you kind of know you’re supposed to know but that didn’t seem to relate to anything when you were supposed to be learning it and to which you therefore paid no attention.