Congratulations - your son has discovered nihilism at the age of nine.
My question is - how would you react if an adult you knew and cared about made such a speech?
Personally I'd find it an interesting conversation to have, to which I would probably disagree with their position thoroughly. My answer to the nihilist question is 'Life is to be enjoyed, and personally I like to make the world a better place, to leave that as a legacy. Even if life is meaningless, we can still enjoy it'.
It sounds like there are two issues here:
How do you deal with your son having these ideas? I suggest getting educated in philosophy so you can discuss them in a constructive manner?.
How do you foster your son's philosophical interests, so he can develop more sophisticated ideas?
The answer to both of these questions, is to read and learn about philosophical ideas.
A starting point I would always recommend, is reading Sophie's World, which is a fictional story that explores various philosophical ideas. It's a very interesting read.
I imagine there are also some very interesting talks on YouTube or Podcasts you could listen to.
A third issue is your son's apparent introversion. I would suggest taking him to a child psychologist is probably a good idea.