Frankly, you may not be able to say anything that will get him to change his mind right now. From what you say, he’s interested only in what’s happening in the present moment, and he doesn’t want anyone to interfere with what he’s doing—including God. His attitude reminds me of the people in Isaiah’s day who said to God, “Keep away; don’t come near me” (Isaiah 65:5).
But you can talk with him about the future—and I hope you will. First, remind him that although he thinks he’s found happiness, he actually is running the risk of great unhappiness in the future. Relationships like this always lack one thing: stability. The reason is because—unlike marriage—they lack true commitment, and can unravel at any moment. What happens if one of them gets bored, or upset, or finds someone more attractive?
This is why I hope you’ll also urge him to face the future, and his need for God’s guidance and wisdom. A life lived only for self is empty and insecure—but God loves us, and He knows what is best for us. Jesus said, “I have come that they may have life, and have it to the full” (John 10:10).
Pray for your nephew, that he will face his need for God’s forgiveness and new life. Pray too that you will be an example to him of Christ’s love—a love that far exceeds the pleasures of the moment.