It is difficult to be sure about your grandson's problem without being able to see him. It is possible that he has a tear of his anus. This is quite common at this age especially if he was constipated up until recently. It would also help to stop all fruit juice and acid fruits such as citrus and tomatoes as eating these foods may cause the burning he complains about. Try a healing nappy cream applied around his anus after every nappy change. Bepanthen is usually helpful for this.
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