Have you ever asked your cousin why he turned away from the faith he knew as a child? It would be interesting to know what he'd say -- if, in fact, he's ever thought deeply about it.
I added that last phrase because over the years I've discovered that many people who drift away from God actually don't think much about it -- or if they do, they end up deceiving themselves. Some, for example, say they've turned away from Christ because they grew tired of their church, or because they decided that Christians were just hypocrites. Others claim they have questions about God that no one has been able to answer, and so they've left God behind.
But often these aren't the real reasons they've rejected Christ. Often the real reason is that they simply want to run their own lives. They don't want anyone -- especially God -- to tell them what to do, and they want instead to live for themselves. They reject Jesus' call to self-sacrifice: "If anyone would come after me, he must deny himself and take up his cross daily and follow me" (Luke 9:23).
Pray for your uncle, that God will convict him of his sin and draw him back to Jesus Christ. Ask God also to make you a witness to him of Christ's transforming power, both by the way you live and by your words. Don't give up; God is still "able to do immeasurably more than all we ask or imagine" (Ephesians 3:20).