The scripture says if you seek you shall find. If there was some way you could get him to pray to God to reveal himself to him God would. We accept Christ because of a personal relationship with God, and if he would actually try to seek God he might receive some insight.
Also, perhaps if you showed him some of the amazing prophecies that have come true in scripture. The book of Daniel, written over 500 years before Christ, predicted the exact year of the arrival of Jesus the Messiah.
Daniel 9 says you are to count 69 X 7 from the command to restore Jerusalem until the coming of the Messiah.
Daniel 9:25, "Know therefore and understand, that from the going forth of the commandment to restore and to build Jerusalem unto the Messiah the Prince shall be seven weeks, and threescore and two weeks: the street shall be built again, and the wall, even in troublous times."
The Hebrew literally says "sevens," translated as weeks. If you count these sevens as years, 69 X 7 = 483 years. The command to restore Jerusalem was given in the seventh year of Artaxerxes reign (Ezra 7:8-13). Historians date that as 457 BC. If you count 483 years from that date, understanding there is no year 0, it comes to AD 27.
In Acts 10:37-38 Jesus didn't begin his ministry as the Messiah until his baptism. According to Luke 1 this event was in the 15th year of Tiberias. Tiberias was ruling over the Syrian province at that time, and according to the Syrian calendar his 15th year was AD 27.
Messiah literally means "anointed one." Jesus didn't receive this anointing until his baptism. Acts 10:37-38 states, "that word, I say, ye know, which was published throughout all Judaea, and began from Galilee, after the baptism which John preached; how God anointed Jesus of Nazareth with the Holy Ghost and with power: who went about doing good, and healing all that were oppressed of the devil; for God was with him."
So, Jesus fulfilled the prophecy in Dan. 9 which predicted 483 years from the command to restore Jerusalem until the coming of the Messiah, which comes to AD 27, the year Jesus was anointed to be the Messiah at his baptism.
Daniel 9:26 states, "And after threescore and two weeks shall Messiah be cut off, but not for himself. . . ." That is after the first 7 weeks; so, after those 69 X 7 Messiah is "cut off." Isaiah 53:8, ". . . for he was cut off out of the land of the living: for the transgression of my people was he stricken."
Dan. 9:24 lists among the things completed as part of these 70 weeks is, ". . . to finish the transgression, and to make an end of sins, and to make reconciliation for iniquity, and to bring in everlasting righteousness. . . ."
Isaiah 53:12, "because he hath poured out his soul unto death: and he was numbered with the transgressors; and he bare the sin of many, and made intercession for the transgressors."
Psalms 22:16-18 states, ". . . they pierced my hands and my feet. I may tell all my bones: they look and stare upon me. They part my garments among them, and cast lots upon my vesture."
In the NT they cast lots to divide up Jesus' clothes while he hanged on the cross.