Usually it is advisable for you to talk with the child privately. Explain to him that when talking--just like when learning other skills--we sometimes make mistakes. We bobble sounds or repeat or get tangled up on words. With practice we improve. Explain that you are his teachers and that his stuttering is okay with you.
By talking to the child in this way, you help him learn that you are aware of his stuttering and that you accept it--and him.