Wow, this is a tough one. One of those stories that makes me grateful for the acne that protected me as a teen, and grateful that my 16-yo DD isn't really interested in boys yet. (Mind you, she did come home from a party last week with what appeared to be a hickey near her collarbone... despite my friendly teasing comments, I got no details!)
I'm getting a feeling like this situation is partly to do with popularity. For whatever reason, this young cad is the "hot man on campus" and every girl wants to be with him to increase her own status. And he wants to be seen with all these girls because - as mainstream media seems to be programming - this increases his status. Maybe you need to figure out a way that dating him *reduces* her status. Or maybe it would be better to find a way that him dating her reduces *his* status - since he's the one in control here. Being socially awkward, I'm not too sure how one might engineer either of those situations, but maybe someone else might know?
Does your daughter have any good reliable friends? If so, it might be worth talking to them to get their take on the situation. Or talking to your DD about what makes this person a friend... a boyfriend should be, first and foremost, a friend. And if her good friends are concerned about this too, then it may help to work together to do some kind of intervention - she may be more likely to listen if her friends are also telling her what you are.
Also, I would strongly suspect that at least some level of sexual activity has been started. Not necessarily "all the way", but at least partway there. The most insidious feature of inappropriate sexual contact is that - physically, in the moment - it feels good, right? Definitely worth doing some sex ed here. Likely she may not yet have come across real useful info about safe sex, sexually transmitted infections, etc. yet. Scarleteen carries lots of info, but bear in mind that it's not anti-sex/pro-abstinence - it's not going to discourage her. But it's real info, presented specifically for teens, and it may get through to her. That particular page is the STI section, but there's lots more. Your message could be... "I don't know if this is part of what's going on, and I don't approve if it is. But if you think you can handle it, then you need to read *all* of this."
It could also be helpful to talk to the boy's parents. It may not help (they may not care, or they may even approve of his "prowess"), but it's worth a try.
And maybe a counsellor of some sort would be helpful? NOT a school counsellor - a psychologist of some sort. Maybe for you. Definitely for her. I agree that self-esteem is likely a part of this.