Just talk to your daughter make sure she knows what it is that is concerning you so much but don't try to push him out of her life, more than likely she already knows she may only have her freshman year with him.
Their will be peer pressure on both sides so she doesn't need the stress of her parents not liking him to add on to it.
Looking back, I now understand that my mom was mostly afraid of me growing up and me dating someone so much older just made her feel like I was growing up way too fast.
Your daughter is going to have to make decisions on her own; ones you don't agree with.
My parents told me that I couldnt date a guy that was older than me and I continued to see him behind their backs.They're young and gaining experience in the world, learning about how people and relationships work.Just do what you, be her mother; be there for her, teach her what you know, and be her support. I agree with the other answers saying that forbidding her from doing anything won't get you very far, but I do understand your concern. He can learn about your family and I think that creates a better open line of communication.Get to know him and figure out their feelings for each other.