Hello !
This is a toughie.  I feel like I need more details to give you a better answer.  I’m wondering what your mom has against this guy.  Is he ‘bad news’ in her eyes?  Has he hurt you before? Is he fifteen years older?  I need a little more info for a fair assessment.  As it is, I’ll try my best.

I am not going to tell you to sneak out to see this guy, and be deceitful to your mom. Small lies turn into bigger lies, and it always gets messy. Besides, getting caught sneaking around with this guy would do nothing but hurt your case. Try talking to your mom, and ask her to trust your judgement. You are a young person who is capable of making up her own mind. Besides, we have all been in those relationships where no one understands the attraction besides the two of you involved with each other. Perhaps she just needs a better chance to get to know him. Is it a possibility for the two of them to meet each other? That might help build the trust between your mother and this guy. She wants to know that her daughter is going to be alright.

I hope this helps. I know it is hard to talk to mothers sometimes, but it is always better to be open and honest each other. I hope it works out for the three of you…   🙂