No, your son is not too young to learn to pray not at all. The fact that he's asking questions about God shows that he isn't too young, and God wants you to do everything you can to encourage this interest.
Remember, just because a child can't understand everything about God doesn't mean he or she can't understand anything. After all, isn't this also true about us? Even if we know a great deal about the Bible and have walked with Christ most of our lives, there are still things about God we will never fully understand until we get to heaven. As the Apostle Paul said, "Now I know in part; then I shall know fully" (1 Corinthians 13:12).
But we can understand that God is real and that He loves us and so can a child. We can understand what it means to trust Him and so can a child. We can understand as well that we can come to Him in prayer at any time and so can a child. Remember Jesus' words: "Let the little children come to me, and do not hinder them" (Mark 10:14).
Ask God to help you teach your son to believe that God is not only real but also that He loves him and takes care of him. (Your local Christian bookstore can suggest a Bible storybook that will help you do this.) Teach him also to pray, simply but honestly. Most of all, let your son see that Jesus is real by the way you live.