I appreciate that there are so many perspectives, but I would keep it simple. It is wrong that she is interested in him- period. They should not be together- period. You are the mom here and you do get to say that they shouldn't be together.
Having said that, it's tricky with teenage boys because, as others point out and you probably know, being too firm and forbidding something will push them closer together. But by no means does this mean they get to run the show and you have to make rules around things you aren't ok with. Just don't come on too strong.
Ask him questions and let him know you are hearing him. What does he like about her? She seems really important to him, tell him you can see that and ask him to tell you about that. Tell him you wish it could be different for him, that you wish he were older or she were younger. But he's not older, and she's not younger. Put some of the blame off of you- it's not just that I'm not ok with this, it's the state of Colorado!
If they end up seeing each other and having sex, he will tell someone, and someone will call the police. Whether or not you end up in any trouble I don't know, but you won't feel like the mother of the year for knowing and not reporting. You shouldn't be put in that position when you are being a good mom and trying to do the right thing.
So in a nutshell- empathize with his feelings, hear him out. But as his parent, you can not and should not let this happen.