Apart from homeopathy, as you mentioned he is a bright boy, my assumption is that he is being bullied at school or you are doing something which has triggered this response.

Evaluate your response to his needs and check out his routine at school. Ask some of his friends (not him directly) and I won't be amazed if you find the answer.

First ask the friends and then the teacher. It could be a teacher's response to him which is triggering this stammer.

An easy test, have him skip school for say a week and you will know the answer or if a vacation is coming up, you will know where the problem lies.

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