There may be underlying problems with the cats gums or teeth that can cause bad breath such as gingivitis which will need to be treated by a vet,so the first thing to be done is visit a vet in order to determine what is causing the problem.
There are several Cat breath freshener products out there on the market today. Some you feed in treat form while others can be sprinkled on your pets food. They are easy to administer and the cats usually love them.
have you had him tested for renal problems? sometimes internal issues can cause bad breath. Also, you can get pet toothbrushes&paste, have you tried to brush his teeth?
