The best thing to do is to make sure he realizes what God is, and that he does already believe in this. The best way to do this might be to read the book "Revelations/Showings of Divine Love by Sr. Julian of Norwich. Other good writings are by Bishop Fulton Sheen, and his television programs, St Thomas Aquinas’ Summa Theologica, especially the first part on what God is and how to know it exists. Another good writing are the devotional hymns by Fr. Faber from England, pictured on my profile, such as his collection “Jesus and Mary”. These emphasize the great love and mercy of God. Once we understand that God is not a supreme being, but Supreme Being Itself, then we will realize that all people actually do believe in him. For example, St Thomas’ 5 proofs do not prove there is some type of giant person in the sky, since that is what an idol is, “We may not suppose, the Divinity to be like unto gold or silver, or stone, the graving of art and device of man.” Believing in a caricature of this kind is something that no reasonable person should be expected to do. But it does prove the existence of existence, the true god, YHWH (which means existence), who is the existence in whom we live and move and be. This great misunderstanding about the nature of God has led to most conflicts about religion, and to people thinking that Thomas’ 5 proofs do not prove God. That’s because they’re thinking of God incorrectly. This the Lord has revealed to me in the last few years, and it has allowed me to see that everything does make sense, and that this God does exist, and that the images of God presented in the bible are just symbols not to be thought of as the reality. This revolutionary understanding (even though it was the original understanding, going back to Jewish and earlier religions) has allowed me to understand everything in a totally new light, and to find out that it was not all made up, as I was starting to believe when I still had the wrong idea of the difference between gods and God. Now I know that truly, as our Lord said, “Sin is behoovable (needed), but all shall be well, and all shall be well, and all manner of thing shall be well.”
"To seek God, if happily (by chance) they may feel or find him, although he be not far from every one of us… as certain also of your own poëts said, For of his kind also we are… And the times truly of this ignorance whereas God disregarded, now he denounceth unto humans that all everywhere do penance (metanoia, or to change your mind to the true understanding of divinity.)