Oh, well, I'm not sure how to actually describe the bad breath that Dax had but I can just say it wasn't pleasant!
Chewy's problem very well could be his diet. You might consider that angle instead of trying to just cover up the smell. I would think there's usually a very good reason for the smell and in Daxton's case it was the rotten teeth - Chewy might need a diet change given that he's getting up there in age. I know Daxton's vet had us change food over the years as he got older - different Science Diet formulas for his health issues and seniority.