What could cause my 6 year olds breath to be so bad even with a normal dentist check up and regular brushing of teeth and tongue ?

Halitosis: Halitosis or bad breath can be caused by many normal things for 6 year old boy. postnasal drip from a cold or outdoor allergies, dry mouth, not brushing well or eating bitter foods can cause bad breath. I recommend that you check his mouth and teeth after he brushes his teeth. Tartar or plaque buildup on teeth or swollen gums - take to dentist. drink water & eat healthy foods. no chewy Candy.