I'm grateful for your concern for your friend's spiritual condition. If we love Christ, how can we be indifferent to the eternal destiny of those who don't love Him (or even scorn Him)? The Bible says, "The fruit of the righteous is a tree of life, and he who wins souls is wise" (Proverbs 11:30).
The first step is to pray for your friend—consistently and fervently. Only God can open his heart and mind to the truth, and only God can convict him of his sins and his need of Christ. Pray also that God will give you wisdom, so you will know when to speak to him about Jesus, and you will know also what to say. The Bible says, "The tongue of the wise brings healing" (Proverbs 12:18).
Pray as well that God will help you be a true friend to him, and that your life will be a living witness to Christ's love. Our lives should be "living sermons," showing others that Christ is important to us, and that He gives us hope and peace in the midst of life's problems. The Bible says, "In everything set them an example by doing what is good" (Titus 2:7).
Then share with your friend what Christ has done for you—humbly and yet clearly. Challenge him also to face the real reasons why he doesn't believe; in all likelihood, he simply wants to run his own life. Most of all, point him to Christ, and urge him to see Jesus as He really is through the pages of the Gospels.