boyfriend’s racist mother

today, my boyfriend told his mom that we are going to prom together. she doesn’t know that we are dating. we have been together nine months. every time he hangs out with me, he says he’s with his two other friends who are girls. she thinks i’m a bitch because i was cheer captain in 9th grade. and she doesn’t want him dating me because i’m black. she got really mad at him and said “great. that’s just wonderful” sarcastically when he told her today. she slammed the door in his face basically. he’s 18 but still living under her roof and only owns a truck partially (he payed some, they pated some). i’m afraid she won’t let him go to prom with me and i’ve already bought my dress. she isn’t gonna want to take pictures of us or help him with anything beforehand (flowers and stuff). what should we do