Frequent urination immediately after drinking water can be normal in some children, especially if this pattern has been present since infancy, the child is otherwise active/gaining weight well, and there is no bedwetting, pain, fever, excessive thirst, weight loss or urinary infection symptoms.
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However, if he drinks unusually excessive water, passes very large amounts of urine, has poor weight gain, burning urine, constipation, or nighttime urination, then a pediatric checkup is advisable to rule out conditions like UTI,
diabetes, bladder habit issues, etc.