My son has smelly armpits what could be the cause he is just 10yrs?

Normal development: The odor producing glands in the armpit awaken under the control of the adrenal glands. This activity can precede true puberty by several years. I would consider it normal at his age.

Bacteria: You son's maturing, yes as early as 10. Suggest daily bath/shower with soap after-shower application of deodorant. The people around him would be most appreciative.

Smell: Usually due to excess sweating Try frequent washing with the addition of a mild deodorant

Hygiene?: Has he been bathing daily? Is he is sports which cause a lot of perspiration? If he bathes daily , you may try applying an antiperspirant to his armpits.