I think he could best be described as nondenominational. He doesn’t take issue with any main stream Protestant denomination, as he is of the opinion that whatever works for the individual is fine. He does dismiss JW and LDS as being ridiculous though.
Is he a sort of " just me and Jesus" type of guy, where church membership is rendered almost utterly useless?
Is he more along the lines of, confused by the numerous interpretations by Protestantism, and think the Truth can not be known definitively?
Sometimes, you may want to point out continuity in faith. What did the earliest Christian writers say on subjects like the Eucharist, Baptism, …etc
What church/churches in modern day, hold to the tenets of faith of the early Christian
How is divine revelation transmitted?
You can ask reasonable questions, that will get him to think about truth claims, through epistemology.
Hard to recommend a certain book, there are many evangelical converts, but at the core of all converts writings is a longing to know the truth.
Many find the Catholic or Orthodox faith through History, maybe that’s a good place to start!
Since the eucharist is the pinnacle of our faith, start there!
Contrast Ignatius of Antioch writings on the Eucharist, with the Gospel according to St. John in chapter 6, Ignatius studied under John, so he is the most connected historically to John.