I know this isn't an easy or comfortable situation -- but has it occurred to you that God actually may have put you there so you could be a witness to your roommate of Christ's truth and peace? Jesus said, "You are the light of the world.... let your light shine before men, that they may see your good deeds and praise your Father in heaven" (Matthew 5:14-16).
After all, what will it take to change your roommate's views? Will it be intellectual arguments that state the case for believing in God, or seek to demolish his own convictions? They may have their place -- but in reality, the one argument he can never answer is the fact of a life that has been changed by Jesus Christ. God loves him just as much as He loves you -- and God wants you to be an example of Christ's love to him.
Remember too that often an atheist who loudly and vigorously defends his disbelief does so because down inside he's actually insecure and uncertain. In other words, he is still trying to convince himself that he's right -- although he probably doesn't realize it, and wouldn't admit it if he did. But God hasn't written him off, and God can get through his defenses.
Pray for your roommate every day, that God will break through his pride and open his heart to Christ's truth. And pray too that God will help you love him and share Christ with him -- both by your words and the example of your life.