Hey, I'm glad your concerned about your friend. Here is what I would do if I was to talk to him. First, tell him that God has given us all free will and he doesn't want us to be His little puppets. So it is our choice to believe or not. The only proof he will get is when he gives God a chance. God will show us that He is real in our hearts if we choose to give Him the chance. And remember to be nice and not to argue.
Show him the ten commandments and then show him this verse...
Romans 3:23
"For all have sinned, and come short of the glory of God;" KJV
Tell him that we all have sinned and that's why we need a savior. Also put in that God doesn't send people to hell. God has given us the choice to be saved, so that clearly shows His love towards us. God has done this by his only begotten son, Jesus Christ. Jesus loved us so much that He gave his life for us. So if we deny Jesus Christ, we are sending ourselves to hell, not God. Also show him verses about hell and heaven and tell him even though he doesn't believe in them, he will be going to one of the two.
Romans 5:8
"But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us." KJV
Revelation 14:11
"And the smoke of their torment ascendeth up for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name." KJV
Revelation 21:4
"And God shall wipe away all tears from their eyes; and there shall be no more death, neither sorrow, nor crying, neither shall there be any more pain: for the former things are passed away." KJV
God is a loving God, but also a just God. He cannot let sinners into heaven, but if we put our trust in the Lord Jesus Christ and repent of our sins, we will be forgiven.
1 John 1:9
"If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness." KJV
If your friend does this, remind him to pray, read his bible and invite him to church. If he doesn't still invite him to church and just keep on talking about God with him. Remember prayer is the best thing you can do for your friend. God is the only one that can soften his heart through the Holy Spirit.
Matthew 19:26
"But Jesus beheld them, and said unto them, With men this is impossible; but with God all things are possible." KJV
I hope this may help, somewhat and I will be praying for your friend. God Bless