They are learning new words at a rapid rate, and sometimes it's difficult for them to coordinate their thoughts, their muscles and their timing. So many children have periods of stuttering from time to time. Most children grow through this with no additional problems, but for others, the periods of stuttering increase over time. When they stutter, they also tend to repeat parts of words like sounds or syllables rather than whole words or phrases. And they frequently repeat those portions of words two or more times before they are able to say what they want.
Some children give exaggerated, prolonged stress to a sound in a word.
Some seem to be stuck with no sound or word coming out. Some seem to work very hard at speaking and others look away just as their speech is disrupted. All of these are signs of stuttering. That just calls more attention to his speech and the added stress is likely to worsen things. Instead be a model for slow speech.