How can i get my mom to let my boyfriend spend the night?

i just dont get it i'm almost 17 & i'v moved in with my bf but didnt like it cuz his place is to small so i moved back home & we are goin on a trip to london together with his family & my mom didnt care cuz i moved in with him or cuz i'm goin to london with him but for some reason she can't stand the idea of him stayin at her house & i told her we would respect her n not have sex in her house n she's leavin state next month for 2 weeks n she said he could stay with me while shes gone how do i get her to let him stay now?