Yeah, unfortunately, divorce ruins most kids’ perception of marriage for life. “I don’t want to end up like mom and dad, so I’ll never ever ever get married to my girlfriend!” And remarriage? Well, people who remarry divorce too. So your son sees what happened to you, and so all faith-based arguments for marriage fail.
I know someone whose parents aren’t married even though they’re pretty moral and pretty devout Catholics, because one of the partners was previously divorced. This person wanted to ensure that they would never go through the pain and financial hardship again and so said “Nope, I will not marry you. End of story.” This couple is still together after many, many years. And there’s a lot of couples out there like that. Kurt Russell and Goldie Hawn are a famous example, they’ve never been married, never will and are completely fine with that and unrepentant about it. Kurt’s scared of marriage because Season Hubley divorced him.
So that sets an even worse example: “Oh man, half of all marriages end in divorce but look at these unmarried couples staying together for decades! Well that’s it, no marriage for me.” The possibility alone of divorce is scary to him.
Unfortunately, like others have said, your son does not care what the Bible says because he does not care about his faith, if he has any at all.
Love is the most powerful factor even moreso, because, “Well, if she doesn’t want to get married, I don’t care because I love her and I’ll do anything to stay with her. Marriage isn’t important, our relationship is.”
Anyone can fall victim to these ways, no matter how devout you think you are. This economy, combined with the reign of cafeteria Catholicism (I know Catholic girls who had kids with their boyfriends, one guy was a devout Born-Again, broke up, and live with new boyfriends and couldn’t care less. They then take these babies to get Baptized. They don’t care at all.), is overwhelming.
Marriage just has too many things going against it now. The way my mother summed it up was, “Why buy the cow when you can get the milk for free?” Only the religious (not just Christians) care. So step 1 is reeling him back in to the faith. Gotta get him to take the faith seriously again. He must understand that God and Jesus are real and they are watching, and they are judging. He must fear the Lord, and he must understand the seriousness of confession and penance.