The average weight for a boy of 6 years is about 22 kg, so your son's weight is below average but not by very much. His weight of 20kg lies on the 25th percentile line of the children's growth chart ; the lowest 25% for boys of his age.All this means is that his weight is definitely below the expected weight for his age but he is not at all seriously underweight.He is eating well.You do not mention his height and he may be following the body build pattern of the men in your family such as being tall and thin. There does not appear to be any reason for concern but it would be a good idea to deworm him just to be sure that he does not have worms.
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