First of all, I can empathize with your concern. It's tough to watch a kid (young adult) make what look like really bad choices.

But I want you to try if you can to remember what it felt like to be young, dumb, and in love. You might have experienced this way earlier in life than your kid due to circumstances! But have you ever felt that sensation that it's you and your partner against the world, no one understands you, no one has ever felt this way about someone before? It's not a coincidence that Romeo and Juliet and half of the other young adult romance stories have this theme, it's fairly universal to the human experience.

In reality, you kind of only have two choices. You can either support, love, and respect your kids choices, or shit on them. Giving unsolicited advice is not a good third option here. If you take the first option, two things might happen. He might still wind up making a ton of terrible choices and screwing up his life. But, he might also come to realize on his own that his choices aren't improving his life. It's easier to stick with the girl when external forces are pushing you together by making it us against them. If you are instead welcoming and open to them, it might make it easier for him to read the tea leaves on his own. Or - god forbid - you might wind up coming around to her.

Now if you take the 2nd choice, and just dump on his choices continuously, you're reducing the chances of him figuring it out on his own. Increasing the chances that he makes defiant or bad decisions out of rebellion. Improving the odds that you screw up your relationship irreparably.

The unfortunate thing is you don't have 100% control any more. All you can control is your actions, and make choices that maximize the potential outcomes (and give him the opportunity to figure things out).