Bad breath may be caused by highly bad teeth, in which case he requires a dental cleaning, followed by home care (such as brushing and eating an adequate diet) from you. The veterinarian may show you how to wash his teeth and give you with safe cat toothpaste.
Animals should never be given human toothpaste since it might make them extremely ill.
If he has particularly foul breath, you should get him checked out by a veterinarian to see out what’s causing it. Bad breath may be a sign of more serious health issues, such as liver or renal illness.