I have a friend who is convinced you can lose your salvation. I have shown him verse after verse, but he is still saying that if you sin you will lose it. How do I show him otherwise?
You might want to give him something to read on his own about the subject. Something he can read in private. That is all that you can do. It is the Holy Spirit who is the real teacher anyway. We are only responsible for presenting the truth – not for convincing someone to believe or understand scripture. The Holy Spirit is the real teacher. Only the Holy Spirit can help someone to believe the truth,
Now we have received, not the spirit of the world, but the Spirit who is from God, that we might know the things freely given to us by God, which things we also speak, not in words taught by human wisdom, but in those taught by the Spirit, combining spiritual thoughts with spiritual words. But a natural man does not accept the things of the Spirit of God; for they are foolishness to him, and he cannot understand them, because they are spiritually appraised. (NASB) 1 Corinthians 2:12-14
If you have been praying for him, continue. If you have not been praying, start. Be available, supportive and loving.