I hope you will write to him, because God brought him across your path, and you may be the only person right now who can give him hope as you point him to Christ. He may have given up on God - but God hasn't given up on him!
What can you say? First, let him know that you were glad to see him after all these years, and you hope you can reestablish your friendship. Also let him know that you're sorry about the troubles he's experienced, and you understand why he feels the way he does. Job in the midst of his anguish sometimes felt the same way: "Why did I not perish at birth, and die as I came from the womb?" (Job 3:11).
Then let your friend know what Jesus Christ means to you, and what a difference He has made in your life. Let him know that you still have your struggles, but you know Christ is with you in the midst of them, and He gives you hope. Make it clear that you aren't writing him because you want to win an argument, but because you care about him. Point him to the Bible also.
Jesus said, "Ask and it will be given to you; seek and you will find; knock and the door will be opened to you" (Matthew 7:7). This is God's promise to your friend, and to all who sincerely seek the peace and hope that come from knowing Christ.