Take a look at the folds around his mouth- are they red and sore? The smell might be coming from there (I have to wipe Murray's mouth area with Malaseb weekly and keep it dry, or he has problems with this)
He might be having dental problems- maybe your vet can take a look-
I agree with Maggie's Mommy. Brushing their teeth really helps. You do need to use a toothpaste made for dogs. We use the rubber bristleded finger cot that fits on your finger. Go slowly at first to get them used to this. The toothpaste we purchase at the vets or you can probably find it on line. Bogie and Bubba, ATB, are the first dogs we have owned who did not have doggie bad breath. We never brushed teeth before them, plus they did not have to have their teeth cleaned by the vet. They are sparkling white.