This is probably not a 'mental' problem, but your nephew is definitely too old to be behaving like this. He will be starting school soon and it will be a big problem for him if he is not toilet trained by then. Give him a complete break from toilet training for 1 week and no one must mention the word 'toilet' to him. Just let him be. Then start again and uses a star chart. He will get a reward if he gets 3 stars in a row, then add one star each time fro his next reward. Praise him if he is successful. No one must be with him when he is sitting on the toilet.
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