How do i go underground and stay hidden?

my boyfriends mother is a complete nutcase and is treating my boyfriend like complete ****. im getting really tired of it because she lies to everyone and makes him look like the bad person. when im 18 i plan on going underground with him and leaving our lives behind. she put him in the mental hospital 3 times, because of things that were not true. she made up stories and had him put in there. with his past, can we get his name changed without it popping up ad his mother finding us? she has MS and her memory is so bad, and everyday is seems like her anger towards him is getting worse and worse. i need tips and advice. thanks!