I believe God is at work in your neighbor's life - and has put you there to help point him to Jesus Christ. Pray for your neighbor, and ask God to give you wisdom as you seek to answer his questions. The Bible says, "Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have" (1 Peter 3:15).
How can you do this? First, make sure your neighbor understands the Gospel: that we are sinners who are separated from God; that God loves us and sent His Son into the world to become the final sacrifice for our sins; that God now offers us forgiveness and salvation as a free gift; that we must respond by believing in Christ and receiving Him into our hearts and lives.
In addition, point him to the Bible; he may never have read it for himself. Encourage him to read passages which summarize the Gospel, such as John 3:16: "For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life."
Finally, ask your local Christian bookstore to suggest one or two books that might help your friend answer his questions. At the same time, continue to pray for him, and share with him what Christ means to you. You may not be able to answer all his questions, but your life will demonstrate Christ's reality to him.