Yes, children do have a way of asking profound questions about God - without even realizing it! But don't let that keep you from answering the ones you can, while also teaching him about the wonder and mystery of God's greatness.
How do you talk about God to a 6-year-old? First, talk with him simply - in other words, in ways he can understand. For example, it's important for him to realize that God created everything - including him. But your son isn't interested in complicated scientific theories or deep theological debates about how God did it, nor could he understand them if you told him. But he can come to realize that God is greater than we are - and because of that, we can put our lives into His hands.
Then teach him from the Bible. Your local Christian bookstore can recommend a children's Bible storybook that retells some of the Bible's main stories in ways he can understand. Teach him especially about Jesus, who loves him and came to open heaven's door for us. Jesus said, "Let the little children come to me, and do not hinder them, for the kingdom of God belongs to such as these" (Mark 10:14).
In addition, be certain of your own commitment to Christ, and ask Him to help you be an example of His love and truth to your son. Our lives often speak louder than our words - and this is especially true as we are with our children.