The little guy has learned that his voice can be used to get attention. Mainly, he uses it to mimic word sounds and to yell at the top of his lungs. Both get attention, but perhaps it is better to give him a little less attention when yelling, and more positive reinforcement of other uses for his voice.
It’s hard, because on the one hand, you don’t want him to be disruptive in public. But on the other hand, the more attention you give him, the worse he becomes, especially in public. This is not an easy one!