B.B.
My niece suffered from chronic bad breath from the time she was a baby. My sister took her to the dentist and everything checked out fine. She brushed her teeth and tongue several times a day but nothing helped. When my niece got older, she started complaining of stomach discomfort on a daily basis. She was still an okay eater and never brought anything up. My sister finally took her to a GI doctor, it turns out she has reflux. Once she started the medication, the bad breath went away.
I would check with your ped to see if he/she has any suggestions for a medical cause such as reflux. Good luck.
-Barbara