Has he been to the vet lately? I mean sometimes it's just bad cat breath from whatever he eats (After Ollie eats his wet food I say he's got "Meow Mix breath") but if it's like really nasty it could be something more serious like a dental or gastrointestinal problem... If it does end up being simply stinky kitty breath, I know there are severl kinds of treats available that are supposed to help and if your cat's cooperative enough, there are pet dental kits (like toothbrushes/pastes or cleaning wipes)... If he hasn't seen a vet recently, I'd personally take him in just to make sure... Oliver was just in for his annual and while Dr. Kathy noticed a little plaque on his teeth, it's not enough for her to be worried... she was telling me she had just gotten back from a conference on pet dental care and is waiting on samples/testimonials of various new products... if I hear any more from her about it, I'll pass on the info
Lets see, Wesley is about 3 or 4. He eats Meow Mix dry cat food. I cannot remember the date of his last check up but I do know it has been in the last year. I cannot remember if we bought him a different food then we normally get because we buy the really big bags that last forever so it was a while in between times that we bought it. He goes outside and sometimes brings us dead mice and stuff. Does him hunting outside have anything to do with his bad breath? Or does that not matter?
My poor Fangy had bad breath for months before I figured to take him to the vet. Poor baby had two cracked teeth that had to be removed. I imagine he must have been in pain, but I didn't detect any. I felt so guilty for waiting so long. Really atrociouis breath is a dental problem. Please take him to the vet ASAP.
As others said, you need to make sure there isn't a health problem.
You could try using PetzLife. I've started using that recently. It's too early for me to say if it is cleaning teeth as well as it claims to, but if nothing else, it should help cats (or dogs) with bad breath.