He probably got it from TV or some older kids. The easiest way to stop it is to step up and not allow it. Next time he does it, tell him "You are NOT allowed to disrespect me like that." Don't argue, reason, or discuss - just send him to his room immediately. He is not respecting you and unfortunately it will only get worse if you don't deal with it. Take heart, best of luck. Remember YOU are the parent in charge! =)
He's doing that because he doesn't see you as a leader. He sees you as his equal, who he doesn't really respect very much.
Where is his father?