Help him learn to verbalize, better. Ask, "Stop screaming and tell me what is wrong? Did Ashley hurt you? Oh, she did? I will talk to her. You sit down while I talk to her."
Or, simply stoop down to his level and ask what is wrong? If he is still screaming, ask again in a calm tone, that will help him to calm down.
Often, children scream when they don't know quite how to handle things, or when they think they are being taken advantage of by another child, and they think no one is going to help them.
You will need to help your son learn what to do if:
-His sister is doing something he does not like. The plan might be to ask her to stop. If she does not stop, then come to me and I will help. Then, you MUST come to help him, or he will continue to scream.
-He can't build his blocks the way he wants to: You will help him to learn better how to do this.
-Someone calls him names: Talk to him and tell him what they say is not important. He is a nice boy and a cute boy and you love him, a lot. "Let's just walk away and never mind this."
-He hurt himself: Go to Mom and ask for help. If he is screaming about this, then say: "Did you hurt your finger? Oh, look at that finger! We will need to get a band-aid for your finger. Oh, look - all better! (Kiss the finger, hug). Now, let's go and play!
Take time for your son, and help him learn to deal with his environment. Take time to talk to him, and help him express himself. Help him learn what will get attention (speaking to you), and what will not.
We have a child in our neighborhood who was a screamer. The parents simply allowed him to do this, and it just continued. He also kept talking in a high pitch, which was very annoying.
Help your son, so he realizes this won't get what he wants or needs, but don't be mean, simply give him better tools. Or, distract him until he forgets what he was screaming about.