You won’t like my answer, but the best thing you can do is do nothing at all. All young children repeat syllables or words a great deal. We do something of the same thing as adults when we fill our sentences with irrelevant words, most commonly, “You know.” Listen to athletes being interviewed after a big game, or people on the evening news describing a tornado. Sometimes we just stick in “Uh” and let it go at that.
Do your best not to tell him to slow down or repeat something he said; just wait out the hesitations and repetitions patiently. If you know what he is trying to say, respond to it as though it had come out fluently. Also, I would teach him a few word games and songs to give him some confidence. Children are less likely to repeat when they know what words are going to be needed.
If it is still a problem after he is about 3½, have him seen by a speech therapist.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.