Huge family issues urgent please help?

My moms fiancé is a drug addicted asshole and I feel really uncomfortable around him and he treats my mom like **** but she doesn't notice it. Whenever I say something to him he insults me and calls me a useless spoiled little ***** and he try's to act like my parent. All he does all day is smoke pot and yell and complain about everything and is really inconsiderate. I swear if he doesn't leave I'm gonna go crazy. Before he gets to out of control with the way he acts with my mom (screaming pushing no physical violence but verbal violence) please ik other people have it much worse than I but I literally can't focus on anything anymore and I'm really afraid of him.