A good question.

He is taking faith seriously now, and is searching for an answer.

Continue to pray for him daily, and be a good role model at home to live out the faith. Prayer changes things. Augustine wandered away for two decades(? exact years?) before he returned to his faith, because his mom nevered stopped praying for him.

Perhaps you can also read the following book with him.

The New Evidence That Demands A Verdict Fully Updated To Answer The Questions Challenging Christians Today (Hardcover)

I read somewhere that kids of Christian parents walk away from the Christianity mostly because their parents are merely Sunday Christians. Their parents do not live a Christ-centered life(e.g. the parents appear to be nice and pious in Church, but once they are home, the parents act differently), and to their kids, they are kind of hyprocrite. We should pray and ask God to empower us with his unconditional love and wisdom so that we know how to bring up our kids God's way.