I wouldn't worry about it. My son asks me questions all the time that I can't answer. Here are some responses:
1. Good question. I don't know the answer. How about you think about it for a while and let me know what you figure out?
2. I am not sure about that. Why don't you write/draw about it in your journal and see if you can create a hypothesis.
3. Maybe there is a documentary on Netflix you can watch on that topic.
4. I have no clue. Next time we are at the library, let's see if we can find a book with the answer.
I don't think having an adult ready with all the answers is necessarily best. I would not stress yourself over this. He can learn to learn things on his own.
