Also, take into account how verbal your child is. My son would scream a lot because he wanted to express an emotion: frustration, anger, fear etc. But, he was unable to express himself with words. He still reverts back to screaming sometimes, but now that he is a bit more verbal, the screaming has decreased significantly. I agree with the others comments about context of the screaming. It's important to know when and why he is screaming. The screaming can be a form of communication, so if you can figure out what he's trying to accomplish by screaming, you can substitute a different word or behavior that is appropriate for the situation.

Really, there are so many screams. Is he happy when he does it? Is he upset? Is he trying to escape from an undesired task? You might have to do a FBA (functional behavioral analysis) to determine when and why he screams

Sensory input of the sound, his way of drowning out the over stimulation of this world. I would take his room and strip it down to a matress on the floor giving him a place this is distraction free that he can go to and not feel like the world is too much. Or is he in physical pain, my little guy had stomach issues and when we switch to the GAPS diet (www.gapsdiet.com) all his problem behaviors resolved.

What is he screaming?? Is he upset or happy when he screams? I know with my son if there is an answer that he doesn't like that we give him or give him good news he will scream this very high pitched scream almost like a screech.

My boy is verbal now, but he still screams when he's excited. He loves "Yo Gabba, Gabba" so I learned the "inside voice, outside voice" song, and sing it whenever he's inside screaming. Or I let him go outside to scream, since he apparently isn't going to stop doing it anytime soon. lol