Cat has chronic URI....anyone have any suggestions on how to treat it?

My 8 yr old grey tabby has chronic URI, his symptoms include uncontrollable sneezing and nasal discharge. Sorry to give the gross details, but basically he sneezes uncontrollably and the cat boogers will fly out of his nose. I'm getting really tired of having to clean them off of my walls/baseboards. He also has horrible smelling breath and his eyes weep. I've taken him to the vet numerous times, the vet always prescribes expensive antibiotics which fix the problem temporarily, but as soon as he's finished with the medicine the symptoms come back. So for a while I've been giving him 2 tsp's of viralysis twice a day mixed in his wet food (its lysine in a gel form) Not so sure its helping him much as he's still sneezing and has the discharge. Is there anything else I can give him that would help with his symptoms? Vitamins etc?