Hi @kglahoda! This is not just something 9 year olds are going to have trouble with. I have trouble with it sometimes! And I sense maybe that in struggling to answer for your son, you want something more solid for yourself. I’m with ya!
To answer your request, Natasha Crain has a great book, Talking with Your Kids About God. It’s really for parents, and “the existence of God” is the first section.
Please allow me, since I have struggled with this myself, to develop a bit the point I keep coming back to: “what is the alternative?”
@Klax is correct and I would go further! By our science we are now confronted with the inescapable fact of the Big Bang, and along with that there must be some kind of infinite (outside our universe), eternal (preceding our universe), immaterial (not made of quite the same stuff as our universe) source that gave rise to our universe. So if you dismiss God, the problem does not go away! Something IS out there which is beyond space, beyond time, and not made of our matter. The question remaining in light of the Big Bang is, “can we know anything more about the infinite, eternal, immaterial?” The Bible claims God has stepped into our reality to tell us about himself.
Part of what the Bible tells us are that God is intentional (He chose to create) and relational (trinity). You can point out that no other species on our planet has these qualities quite like we do (other species adapt to their environment, we adapt our environment to ourselves), and the Biblical claim that God endowed humans with some part of himself has a rational logical flow to it. Humans since time immemorial have believed these properties in us came from some kind of deity. This provides a much more sound ontological foundation for those properties in us than having them “emerge” from matter. If there is no god, they would ultimately be just illusions. So choices, love, truth, meaning, reason, friendship, etc. would be all mechanical extensions of matter that improve survival rates, neurons firing providing an illusion of these things with no knowable relationship to objective reality. In contrast, the testimony of our own inner being tells us “there is something real in these!” And this aligns beautifully with what the Bible says about God and us. This is like a CS Lewis quote (roughly), “I believe in God like I believe in the sun - it’s too bright to look at, but by it I see everything.”
Taking a step upward from this into philosophy, the more basic question is why anything exists at all. As Francis Schaeffer pointed out, what we’d expect is that nothing would exist and no one would exist to know that nothing exists. That’s what we’d expect! Existence itself is the truly big mystery. We’re stuck trying to figure out something useful when all categories of answer are impossible for us to truly fathom.
Well, these are a few late night musings. God bless you and your love for your son, to seek out more meaningful answers to his questions! I hope that something in the responses proves helpful for him, and maybe also for you.