Your acquaintance probably considers himself a very logical person, and he may even say he doesn't believe in God because faith doesn't make sense. But if God doesn't exist (as he claims), then why does he get so upset whenever anyone mentions Him? Why get upset over something that you're convinced doesn't even exist? He isn't as logical as he thinks he is!
In other words, I suspect that down inside this person knows that God exists—even if he won't admit it. And he won't admit it because he doesn't want anything to do with God, nor does he want God to have anything to do with him. He is determined to live his life on his own, without anyone telling him what to do—including God. He wants to be independent and in charge of his own destiny, and anyone who believes in God is a threat to him.
But the Bible calls someone like this a fool: "The fool says in his heart, 'There is no God'" (Psalm 53:1). Why is such a person a fool? One reason is because we need God's wisdom and help to choose the right path in life. Another is because someday this life will be over, and he will be unprepared to meet God.
Ask God to help you be a living demonstration to this person of Christ's love and compassion. And pray for him also, because God is able to break through even the hardest shell people put around themselves.