Let him know you think your mom will say "no" ... and tell him to give her reasons why she should say "yes". I don't mean just "I love her" - but how he plans to provide for your/his future after marriage. Maybe once she is satisfied he has thought out all the important details beyond love, she will say "yes".
Forget mom. I was battled my first time similar to what you are except my wife was sixteen, didn't HAVE to get married just wanted to. Lasted eighteen years then the bottle came between us. Did find a great replacement .
From your question it seems the boy is very straight forward and he don't want to have any trouble in his affair with you. He is thinking for a longer and permanent relationship rather then the short one which now a days is very common. And I am damn sure he will convince your mother, he just want your mother to know that he is going to make friendship with her daughter. You all the three should along with your father sit and decide on further course of actions.
It really is about showing his respect for your mom and is she says no ,but your an adult who takes care of herself and isn't dependent on her mother any more I don't see why you would have to follow what she says unless you want to. It's nice of him to do this and she might see this.