I'm not going to let my mom's boyfriend stay
Why wouldn't you want him to stay?
He scares me and I don't want to be home when he is there
If he is making you uncomfortable, you should speak with your mother about it. You deserve to be safe in your home.
I'll convince my mom to kick him out and put her in a position to either pick me or the boyfriend.
If you get him kicked out your mom will be angry at you. It's better to talk to her and explain how you feel over just immediately demanding he leave. At least she will be mentality prepared. Try your best to work out a compromise instead?