Hi, I'd like to help you with your question.
It is understandable that you are concerned for your daughter. You want her to be safe and if this boy seems suspicious to you, then expressing your concerns to your daughter is certainly a good thing to do.
However, although you do want to keep an eye on the situation, your daughter is going to have to judge this relationship for herself. Unless you have any direct evidence of her being abused, stalked or otherwise hurt, she is free to see this boy. Now that does not mean you are not going to worry. That is understandable. It also does not mean there aren't things you can do to help her.
One, talk with her about your concerns again. Let her know that you suspect there may be more to this boy than meets the eye. Tell her you are concerned, but that you know she is an adult so it is up to her as to what happens with the relationship.
Two, let her know that you are there for her anytime. Tell her to call day or night if she needs to. Do not give her scenarios like, "if he hits you call me" but leave it general when you make the offer. You do not want her to feel you don't like her boyfriend and that you are alarmed about the relationship. If she feels you might overreact, she may not be willing to turn to you if something does happen.
Three, stay in touch with her as much as is reasonable. Visit her at school get to know her other friends as well. Do not go behind her back and talk to her friends about her relationship but knowing other people in her life is a good way to help her. Her friends may feel safe contacting you if they see something out of sorts. Just be sure to maintain your daughter's trust.
Otherwise, you will have to let her make her own decisions about her relationships. She may make mistakes, but she will learn. And with supportive parents, she will always have a backup if she does have problems.
I hope this has helped you,