Hi. I'm Dr. Lee and will do my best to help.
Does he eat a lot?
Any increase in urination? Increase in urine clumps in the litter box?
How long has the sneezing/coughing been going on?
Thanks for the info.
I would suspect there is one of the following problems going on: hyperthyroidism, diabetes, kidney disease, liver disease, and cancer. These all lead to similar clinical signs as you have described, especially the increase in water consumption and urination. I would highly suggest to take him to your vet for exam, bloodwork, and urinalysis. These are the bare minimum tests needed to determine the underlying culprit, so the proper treatment regiment can be prescribed.
Please let me know if you have any questions or concerns. Thanks.