I'm thankful your son wants to know about God, and I hope you will do everything you can to encourage his spiritual interest as he grows. May your prayer be that of Samson's father in the Bible: "O Lord ... teach us how to bring up the boy" (Judges 13:8).
Your son is now old enough to realize that everything we see around us was created from something else. In other words, the things around us didn't just happen; they had a beginning, and someday they will also have an end. And that's why it is hard for us to imagine something that has no beginning and no end.
Yet this is exactly the way it is with God. God had no beginning, nor will He ever come to an end. No one created Him; unlike everything else, He has always existed. As the Bible says, "God ... is from everlasting to everlasting" (Nehemiah 9:5). The Bible also says, "I the Lord do not change" (Malachi 3:6).
This is an important truth — and one reason is because it means we can trust our lives into His hands and know He will never change or turn His back on us. He made us, and He loves us so much that He wants us to spend eternity with Him in heaven.
How do we know this? We know it because the eternal God entered time in the person of His Son, Jesus. May you and your family put Him at the center of your lives.