My brother is very unstable...how can I help him?

My brother is 21 years old. He lives at home with very loving...yet slightly over protective parents. He's so unstable...nobody knows which Scott they're going to be dealing with. He goes from being happy and joking around and laughing...to SCREAMING at everyone telling my dad to shoot him and call the cops on him if he wants him out of the house, he smashes his cell phone and slams doors, he grabbed my mom by the hood of her sweatshirt and pulled her backwards up the stairs, he slams doors constantly. He's just so up and down. I'm scared for him and my family. He's having a very difficult time right now. His truck is broke down and he just got layed off. I don't know if he's really depressed or if he has a mental problem. It's to the point where nobody wants him to live there anymore because all he does is freak out on us and make everyone's life miserable...until he's the good Scott and playing with his little brother and laughing at the Simpson's movie. Please help.