My boyfriend and I want to get married, but...?

his mom got married young and then got divorced so she has a bad view on it. We're both 18 and have been going out for a year and a half. My parents know how I feel and they've accepted. My boyfriend doesn't want to tell his mom yet because we both need to go to college and he doesn't want his parents to kick him out if he proposes. We're waiting to have sex until we're married so living together is not an option and we were planning on having a long engagement. How do we tell his parents without them completely overreacting?