Speak to your child in a normal tone of voice. Duh!
Your child is most likely to yell when he is trying to get your attention and you are doing something else. Help your child understand that it’s not appropriate to interrupt you when you’re talking to someone else—that he simply needs to wait or ask you what he wants to ask you in a normal voice.
Teach your child the difference between quiet, normal, and loud. Say, in a whisper, “This is a quiet voice.” Make him repeat it in the same tone. Then say, in a normal tone, “This is a normal inside voice.” Have him repeat it back. Then say, in a loud voice, “AND THIS IS LOUD!” Have your child repeat it. Do this over and over until he understands the differences. You can even have him sing a favorite song in a quiet voice, a normal voice, and a loud voice.
When your child yells, don’t tell him to simply “stop yelling.” Say, “You don’t need to yell—I can hear you when you use your normal voice.” Teach your child when it’s appropriate to be quiet (e.g., when his brother is napping), to use a normal voice (e.g., in a restaurant), and to use a loud voice (e.g., when he needs help).