All children in this age group are busily learning to talk. As such, they make speech mistakes. We call these "mistakes" disfluencies. Some children have more than others, and this is normal. There are certain children, however, who have many disfluencies--particularly repetition and prolongation of sounds. These are quite noticeable to listeners.
If you are concerned that there may be a problem of stuttering developing with one of these children, don't pay any special attention to the child at this point. Rather, talk to a speech pathologist for suggestions.
Also, talk to the parents about their opinion of the problem so that you know whether this is typical speech behavior for him. In most instances, if parents, teachers, and others listen to and answer the child in a patient, calm, and unemotional way, the child's speech returns to normal as his language abilities and his adjustments to school improve. If the child continues to have disfluencies, however, you may want to ask a speech pathologist to observe him.