Many people, I suspect, think of God like you do–as a kindly old man with a beard. Or they may have some other picture of God in their minds, imagining Him like a stern policeman or an angry judge.
But none of these is accurate–and the reason is because God does not have a physical body. God, the Bible says, is a spiritual being, and as such He has no physical appearance. The Bible says, "We should not think that the divine being is like gold or silver or stone–an image made by man's design and skill" (Acts 17:29). God is far greater than we are–in fact, He is far greater than the whole universe. The Bible says God is the one "who alone is immortal and who lives in unapproachable light, whom no one has seen or can see" (1 Timothy 6:16).
I know this is hard for us to understand–and I know it's also hard to explain it to a child. But ask God to help you teach your son how great God is–and also how much He loves us. He loves us so much that He sent His only Son into the world to forgive our sins and take us to heaven someday.
As a footnote, let me suggest you buy your son a Bible storybook, which retells some of the Bible's stories in a way children can understand. You might read one story with him each night, and then pray with him before he goes to bed.