Should I stay with him?

You're 18, which means you're allowed to make your own decisions. Your mom has no say in the matter anymore. However, before you take another step in the relationship, try to hear your mom out and find out why she doesn't like him. Sometimes there's a reason parents don't like their child's significant other and it's because they can see something that the child might be blind to. Don't take it as she's trying to ruin your life. She loves you and doesn't want you to get hurt. You should try to explain to her why you think your boyfriend is good and ask her to give him a chance. Ask your mom to have dinner with you and him. That will give her an opportunity to see a different side to him. I'm not sure if you've done all this already, but if you haven't then this is what I feel you should do. Hope this helps.