Bad breath in cats usually has four causes: diet issues, dental, parasites or gastric problems. I suggest you check the following:

Diet : some canned food can cause bad breath in dogs. You should check your pets’ diet. It has to contain solid (hard) food to help with cleaning the teeth. If he is on a special diet for some reason consult your vet before changing his diet.  Also you might want to try the artificial bones for teeth cleaning. It is important that your dog’s diet is according to his size and weight, level of activity, special needs (current conditions). An appropriate diet helps in keeping your pet healthy.

Dental problems: your pets mouth should be checked at least once a year. In your case if there is an odour problem, a dental check is the first thing to do. Diseases like tartar buildup, gingivitis, periodontist, mouth tumor can cause bad breath. Also a foreign body stuck between its teeth can cause bad breath.  Usually common problems can be treated during a single session of dental treatment. It might be helpful to start brushing his teeth if you are not already doing it. If you can to do it every day it can prove helpful and it can solve your problem. There are some professional toothpastes and toothbrushes.

Gastric problems: sometime if the diet is not according to your pet’s needs, it can cause gastritis which also leads to bad  breath.

Parasitic problems : Parasitic infestations can also cause bad breath