It is unusual for a 9 year old boy to have smelly armpits. You mention that you have noticed this for the past 2 years which means that he was 7 years old at the time. As this is usually a sign of the onset of puberty and your son was only 7 when it started he is obviously too young to be entering puberty already. So this is known as early or 'precocious' puberty. Your son should be seen by a paediatrician about his precocious puberty as he may need to have tests done. In the meantime he should use a low allergy alcohol free deodorant under his arms after bathing or showering.
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