If you are not at least 19, your mother is absolutely right.
I guess you've just got to make that decision for yourself. If your mother believes that about him perhaps she has reason to? Think about it and, whatever choice you make (to dump him or stay with him) accept the consequences. x
She's probably right, and your lucky she's nice enough to not deck him in the face. Best believe if some 21 year old dude showed up on my steps for my teen daughter, he'd be meeting the business end of my 12 gauge.