Hello, Kim
Well you have done everything right and so has your vet. I too think this sounds like feline herpes given the chronic symptoms.
The best thing anyone can do for a cat that has feline herpes, is boost the immune system. You can do this by giving him L-Lysine. You can find this at drug store that has a good vitamin department or GNC type stores.
Boosting the immune system suppresses the virus, thus easing the symptoms.
You have tried and done every thing imaginable to control the sneezing and congestion except for using a decongestant nasal spray like Afrin. Afrin is safe to use in cats. Just one spray in each nostril twice daily. I would keep up on using the Chlor-Trimeton twice daily as well, and any prescriptions your vet has given. Try a vaporizer with some Vicks in it too. This can help thin out the mucus. It works better than just plain steam.
Get that immune system up though. That is the most important part. This is a frustrating condition for any cat owner. I hope it gets better for you and him.
Sincerely, XXXXX
Expert in cat behavior and health
Expert in cat medical and behavior issues.
I do have one question. Is he currently on the Zithromax? If so, how long?
If not, how long did he take it before?
Thank you for that additional info. Sorry for the delay in getting back to you. My computer decided to freeze up on me :(
Another possibility of these problems could be a bacteria called Bartonella. Another expert here on Just Answer advised me that to treat this, it just takes 21 days on Zithromax. That is why I asked. It is a stubborn bacteria and it takes more than twice as long on this antibiotic to treat. I would discuss this with your vet. He/she may have already tested for this and found not to be the case, but just thought I would mention it.