Screaming kid won't stop?

About a year ago, my sister, her deadbeat husband, and their kids moved in with me because he can't/won't hold down a job and they can't afford their many children and keep their glamorous lifestyle at the same time. Anyway, tensions are running tight between us for several reasons, but mainly because of her youngest child. The kid will not stop screaming. He screams when he's hungry, he screams when you feed him. He screams when he has a messy diaper, he screams when you change him. He screams if the toy he wants is on the other side of the room, if he's bored, or if it's too quiet. In fact, in the short time it has taken me to write this, he has screamed on five separate occasions. They have taken him to the doctor and nothing is wrong with him. Please, before I end up on the six o'clock news, tell me why he cries constantly.