There are a number of causes of pleural effusion (fluid building up around the lungs but not in the lungs) in cats. This build up of fluid results from changes in the way the blood and other fluids in the body are transported. This build up of fluid does not directly effect the lungs, but reduces the amount of space for the lungs to inflate...thus not allowing the animal to breathe correctly. Based on analysis of the fluid and to look at the cells that are in the fluid, many times we can more accurately determine the cause.
Here are the causes of pleural effusion in cats:
Cancer (typically a mass/cancer either in or around the chest)
Chylothorax (this is a milky white fluid caused by obstruction of the lymph system)
Pyothorax (this is pus int eh chest from infection)
FIP (Feline infectious peritonitis, a virus)
Hemothorax (blood int he chest from trauma)
There are a few other ones that are not nearly as common. With your little guy, the top three that I would consider would be heart disease, FIP, or cancer. Unfortunately, none of these are good diseases to have, but they are manageable to an extent. As for the blood work, in these particular disease, the blood work can look totally normal until the disease gets really bad.
In many cases. Having the fluid drained off makes them do great for a while. I have seen a couple of cats managed for a year or so that may, but I would do a little more diagnostics and see if you can treat the underlying disease and make this guy feel better for longer!
I hope that helps, please let me know if you have any more questions!