What is the best supplication or verse to open the heart of a loved one to Islam?

The best course of action in this situation is to supplicate for his guidance. This is what the Prophet, peace and blessings be upon him, did, as narrated in the hadiths of Tufayl ibn Amr Al-Dawsi, and Abu Hurayrah concerning his mother, and his supplication for the tribe of Thaqif, where he prayed for their guidance instead of praying against them.