I commend you for pointing your friend to the Bible, and I encourage you not to abandon this effort, even if he doesn't believe it is God's Word. Over the years, I've found that the Holy Spirit often uses the Word of God to penetrate even the most resistant heart. As the Bible says, "The word of God is.... Sharper than any double-edged sword" (Hebrews 4:12).
At the same time, the most important thing you can do is urge your friend to discover Jesus Christ as He is revealed in the four Gospels of the New Testament. He may have doubts about the Bible -- but has he ever honestly examined what it says about Jesus? Jesus claimed to be the divine Son of God who came to earth to save us from our sins. Is His claim true? Urge your friend to investigate that question for himself.
The two most important questions any person will ever answer are these: Who was Jesus Christ, and what difference should He make in my life? Make sure of your own response to those questions -- and then challenge your friend to face them, as well.
Most of all, pray for your friend, that he will realize his need for Christ and turn his life over to Him. When he does, the Bible will come alive for him -- and that will be his proof that it is indeed God's Word.