We expect and accept that. These are like speed bumps in the child's talking. They often occur when he is excited or talking rapidly. This slows his speech, but also robs it of smoothness. Talking requires timing skills and the coordination of the many small muscles of the lips and tongue.
Young children are beginners at talking.
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.