My cat Hunter had really bad breath once too and it turned out that his tonsils were infected. Is your cat making any licking movements with his tongue or shaking his head and stretching his neck? When my cat had this happen, he did not like me to give him any under the chin rubs like he always enjoyed in the past.
Also, he was not eating or drinking normally either and soon began to throw up a lot. At first the vet thought that Hunter had swallowed something or a blade of grass got stuck behind the soft palate. We even took xrays to be sure, but nothing showed up, like a needle or something.
Upon examination my vet said he could smell the strong smell of infection in Hunter's tonsils so he was on antibiotics and IV fluids injected under his skin. He became really sick. Now I am always sniffing his breath just to make sure I smell nothing fowl. If I were you, based on what happened with my cat, I would take him in for a checkup just to be sure that there is no infection starting up so you could catch it early. As I said, my Hunter was VERY sick and had I taken him in to see the vet the first time I noticed his breath was bad I could have avoided him getting so sick.
Good Luck
