Possible cause of cat runny nose

Question

My cat started having problems a few years ago. He started having a constant runny nose and sneezing with a lot of discharge. He also started vomiting frequently. I took him to his vet a few times, but they never found anything. They just told me to try changing his food and try allergy medicine. Changing his food helped with vomiting (it only happens occasionally now and will stop if I change his food again) but did not nothing for the runny nose and sneezing and the allergy medicine did nothing. I found some cat sinus relief medicine online that has been helping. The runny nose and sneezing has been cyclic since starting him on this medicine. But a few months ago it came back bad and is getting worse. He also started losing a lot of weight. Any ideas what could be causing this? The only thing I can think of is an bad tooth causing reoccurring sinus infections.