Halitosis, or unpleasant breath, can be caused by a stomach problem, bad teeth or tonsils, or something that is stuck in the dog's mouth or throat. Look your dog’s teeth to see if any are decayed. If not, then give him 3/4 teaspoonful of bicarbonate of soda in his food twice a day for a few days. If his breath is still offensive, then take him to a vet to see if he needs his teeth cleaned or his tonsils treated.