Hi John: The human mind cannot conceive logically of any effect that is missing its cause. Teleological/cosmological arguments both focus upon that axiom.
Usually I go straight for the elephant in the room.
If I offer a real-world example of cause and effect such as gravity or wind, the gravity causes the 'downward motion', the wind causes the 'effect of air' upon the subject. No one of sound mind denies any of that. Simple stuff, but then comes causation for morality (moral argument), causation for logic itself ( Transcendental argument)
Intangibles' (logic, morality) have causation as well, the usual answer from a materialist ( someone who believes there is only the material and never the spiritual) has existence and therefore initiates causation will answer with 'mental constructs' or with 'group think' whereby the tribe agrees to what is acceptable and what is bad.
The examples I gave are for you to clear away any cobwebs you might have...the elephant in the room is the denial of God's creative workings whereby man is made to 'understand spiritual things' and man is made to know God himself.
When you see this denial of existential things over against acceptance of material things, you are witnessing the human heart simply 'denying what's logically necessary' in favor of what is 'humanly pleasing to a person that is happy God is not interfering with their life'.
The real question once their denial of logic is evident is to ask "What is it about God that makes you want to deny him"? That is where the real and much more substantial conversation needs to be had.
The other aforementioned arguments are really just to flush out the fox from the hen-house and get to the crux of the problem which lay in 'why he is denying God'.
As soon as he cops to offering an answer to 'why' you no longer need to offer 'proof' for God because he has already confirmed it to you.
If he decides its too dangerous to talk about his real issues with God, he may run back to 'well there is no evidence for God' nonsense when in fact he's retreating for cover.
If you deal only on an intellectual level you will always stay in the dis-honest portion of his life. Intellectual arguments from unbelievers are always for preservation of the sinful-CONDITION of the heart, it's never to offer up pure logic rebuttals.
If you really want to be dangerous and exercise faith, tell him "you watch my life" I am a servant of the living God, a Son of God and a Brother to Jesus Christ who loved me and redeemed me by his own death. Challenge him to consider your own walk with God as a living breathing 'proving' that God lives and works in my life.
some food for thought...and yes pray for wisdom, boldness and courage to speak clearly and directly to him.