Marriage Requirements
Well, she can't marry him without your (or her father's) say-so unless she emancipates herself. Which would be tricky with a baby, no means of support and still having to go to school (hopefully).
That said, remember that he's a dumb kid. She's a dumb kid. They're doing dumb things. But were you much smarter at that age? It'll be tough, but try to stay on the high road. You've done well so far.
If I were you, I'd make it a point to reach out to his folks to see what they have to say. You're going to share this grandchild, and if nothing else, it would be good to get on the same page about that.