L.S.
Your little one is discovering language! He understands that when he "speaks", people listen! Now might be the time to introduce the concept of "inside voice" versus "outside voice"... try whispering to him while in the stores... make it into a game, or find a great place where it is "legal" to shout, or experiment with sound at home: yell into a box, a paper bag, a metal bowl, whatever to see how it echoes. At the same time you might validate to him how fun it is to use a big voice in a big (and echoing!) store, but that people in the store don't like those noises (although I am certain that people are smiling when he yells... that part is hard to fix). You could always go with logical consequences: If you continue to yell we will have to leave the store... then DO it! Messes up your shopping plans, but when done consistently makes a clear point. Another way is to go shopping without him, then talk to him about how you missed his company, but not the yelling. Good luck!