Ohhhh. How sad. Heartbreaking!
If he’s smart, as you said he is, there are big chances he’ll return. If he wants to find the truth, he will. He’s by law an adult and you can’t stop him from doing what he’s doing. If you’re close with him, maybe you should tell him something like, “It’s your life. Do with it whatever you want. You have a bechira. You’re a smart man. You’ll see there’s really nothing there. If you davka want to go see for yourself….Well, I won’t stop you. I really think it’s a pity you chose to live and empty and pointless life. Remember, you can always come back. No matter how low you stoop.” May Hashem guide him in the right directions.