One thought that I would work into the reply would be in response to what he said about "making sure of the more important things", God and Christ, and such:
Mention to him that part of "making sure" of those important things would be to heed the counsel from God and Jesus as outlined in the Bible. For instance, we can't ignore what Jesus said at Matthew 24:23-24 about those, like the WTS, who claim that Jesus returned invisibly. Jesus didn't say, "Be on the lookout for people who claim that I have appeared, and then years later, change their teaching on it. That will be the true religion. Even though they were wrong, they have my blessing." No, Jesus WARNED us to watch out for very things that the WTS has done. Even in verse 25, Jesus emphasizes it again by saying, "See, I have told you beforehand."
Help him to understand the confusion that he might be feeling, and that our Creator helps us with that through the scriptures, too. Share Deuteronomy 18:20-22 with him concerning that. God makes it clear in those verses that any man, group, organization that prophecies/predicts things claiming that they come from God, and they DON'T come to pass...God assures us that this man/group/organization is 1. false, 2. will die, and 3. is nothing to be afraid of.
In light of those verses and God's strong disgust toward false prophets, it would be logical to accept that the verse in Proverbs 4 about the "light getting brighter" was NOT included in the Bible canon as an excuse for false prophecy. God's revelations to His people throughout the centuries have been clear, complete, and backed by evidence of His spirit. And individually, as Solomon said, the light DOES "get brighter" for us as we progress in our understanding of God and the Scriptures.
Hope my input was helpful.