One thing I'd probably tell him is that he would only be hurting himself by doing this even if he doesn't realize it right now.
Why do I say this? The reason is because this kind of relationship provides a very weak foundation for his future marriage. You see, the one thing that's missing in a relationship like this is commitment. At any time, either one of them could simply walk away, because neither of them really would be committed to the other person. But if they don't take marriage seriously now, what reason is there to think they will take it seriously once they are married?
Marriage involves commitment a solemn vow of a husband and wife to be faithful to each other for the rest of their lives. This vow isn't just between the two of them, but between them and God. This is why the Bible says, "For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh" (Matthew 19:5).
But I'd also want to point out to him that an even larger issue is at stake and that is his relationship with God. Right now, he is turning his back on God and wants nothing to do with Him. But he needs God, both for this life and the next. Pray for your son, that he will face his sin and open his life to Christ.