Upper respiratory infections can cause bad breath but usually not the only reason. You can try "Feline Greenies" as treats to help with the teeth cleaning if he wont allow you to brush (although I would continue to try that as he may eventually get used to it). For any nasal congestion or just even to break up dried mucous get "Little Noses" in the baby section at any pharmacy/supermarket as its sodium chloride and will break it up much easier than water. Just put a drop or 2 in each nostril (he wont like it). I always used a baby nasal aspirator also if I found lots of congestion just to clear the airway. Do you have a humidifer or vaporizer? My vet even told me to use Vicks vapor rub in the vaporizer (be sure kittie doesnt get it on his fur or nose and out of his reach) and run it near his sleeping area. It really helped my little one! AD ointment or vaseline (figure petrolum is in all the hairball meds so not toxic in small amounts) would be better than neosporin as if he's cleaning/licking his nose he can ingest the antibiotics that are contained in the neosporin (also not made for ingestion). A little may not hurt but it can desensitize them to times when they do need antibiotics. I am always open to new ideas also if anyone else has anything to add.