My nine year old son mouth smells bad breath with no dental problem what is the cause.

Bad breath: There are many dental and medical causes. If you're dentist feels there is no dental cause, take your son to a pediatrician. For some basic information on this subject see: http://smilesapartcosmeticdentist.Com/fresh-breath-control.Htm.

Tonsils: Could be tonsils/adenoids. Ask your physician to check. N could also be from not cleaning between teeth. Need to floss daily. See your dentist.

See a pedodontist: The proper diagnosis is the key to a successful treatment. See your pedodontist for Halimeter test, diagnosis and bad breath treatment that depends on its cause. Good luck.