I don't know if he's sincere or not; he may not even know himself. Sadly, some people today are like the ancient Athenians Paul met, who "spent their time doing nothing but talking about and listening to the latest ideas" (Acts 17:21).
But even if your friend isn't sincere -- or even if he only sees God as an interesting topic rather than the divine and holy Creator of the universe -- I still encourage you to share your faith in Christ with him. After all, God loves him just as much as He loves you -- and God is able to break through the shells of indifference and rebellion that people erect around their souls. Pray for him, that God's Spirit will open his eyes to the truth -- both about himself and about Christ.
Let me urge you also to share with him what Jesus Christ has done for you. You may not be able to answer all his questions -- but he can't deny the reality of what Christ has done in your life. And if you find this is hard to do, perhaps God is speaking to you about your own need to walk more closely with Him every day.
In addition, ask God to give you the boldness to ask your friend if he's ever faced his own need of God. He may believe he's self-sufficient and doesn't need Christ -- but he does. He needs Him both for this life and for eternity. May God use you to help him realize this truth.