Young people often question their faith as they grow older, particularly when they get away from home and run into people who reject God. Some of our own children experienced this—although in time God brought them back to Himself and they committed their lives fully to Christ.
Did you fail somehow as parents? Perhaps—but not necessarily (although none of us is perfect, and if you know you did fail, ask for God's forgiveness). But don't spend all your energy worrying about how you've failed, or thinking about what you could have done differently. The main thing is to look to the future, and ask God to show you ways you can encourage your son to make Christ the foundation of his life.
What can you do? First, pray for your son. He faces many pressures and temptations, and he needs God's strength to resist them and avoid the moral and spiritual dangers he may be facing. Pray too that God will give him a hunger for God—a hunger that can only be satisfied by Christ.
Then urge him to seek answers to the questions he is facing. He isn't the first person to ask them, and across the ages Christians have found answers to them. Most of all, encourage him to discover Jesus for himself, for only in Christ "are hidden all the treasures of wisdom and knowledge" (Colossians 2:3).