No person who is employed has any right to verbally abuse the person who is employing him. None. If he "flies off the handle" when you tell him about the pay decrease, tell him "This is unacceptable. If you continue to speak to me in this way, you are going to be asked to leave."
If he's really stupid as well as mean, he'll say, "Okay, I'll just leave, you so and so, I quit!"
But if he won't quit, and doesn't pipe down, tell him that you are terminating him for his behavior. If the behavior escalates when that happens, call the police and have him removed from the premises.
It is wrong to subject yourself and the other employees to a bully in the workplace, especially when YOU are paying HIM to work for you.
Do not listen to him rant and threaten lawsuits. You actually can fire him "at Will." Under these circumstances, he can file for unemployment insurance, but likely will not even be approved for that. DO take it seriously if he makes serious threats, such as "I'll come back here and blow your head off." Don't be dumb. Call the police, get protection from him.