I am sorry to hear that Fritz is feeling so poorly.
Even though he seemed to get sick overnight I suspect that the fluid was slowly developing and he was able to accommodate the decreased lung function and oxygen exchange for a period of time until it hit a critical point where no matter what he did to ease the stress on his body (resting more, playing less or active for shorter periods) his brain and organs were no longer getting the appropriate levels of oxygen, his symptoms of struggling to breathe became obvious and that is when you noticed something was wrong. This is extremely common with cats. Being small it is in their best interest not to show signs of weakness and they learn to accommodate an illness as long as it isn't a sudden change.
Once they get to the point where they really can't breathe and oxygen exchange is poor the body shuts down the appetite center in an attempt to focus oxygen exchange in the brain and heart, shunting oxygenation away from the abdomen/gastrointestinal tract to keep the essential brain and heart oxygenated.
A high white blood cell count simply means he has an inflammatory process. That can mean a tumor or an infection.
If your veterinarian was checking him for FIP I suspect that the fluid is actually accumulating around his lungs (pleural effusion) such that the lungs are compressed and unable to fully expand and fill.
Has the fluid around his lungs been aspirated and analyzed? That is important to help diagnose his condition, which would help direct treatment. Removing some of that fluid may help him breathe easier for a short period of time while a diagnosis is being achieved.
I would ask your veterinarian if the fluid is around his lungs (pleural effusion) or in the lungs themselves (pulmonary edema)?
Is the fluid clear in character, or cloudy?
If the fluid is milky then bacterial or fungal infections or chylothorax would be possible reasons for fluid in his chest.
Clear fluid around the lungs can be secondary to heart disease, inflammation due to a viral infection (primarily FIP in cats) or due to a mediastinal mass such as a thymoma (thymus gland tumor), or lymphoma (cancer of the lymphatic system). Both thymomas and lymphoma are more commonly seen in cats with Feline Leukemia. If he has not been tested for feline leukemia and feline immunodeficiency viruses he should be.
Ideally the fluid should be sent out for analysis. The most common types of cancer that lead to pleural effusion (either lymphoma or a thymoma) don't tend to shed cells well, so we cannot always find cancer cells even when a tumor is present. And with fluid in the chest visualization of a tumor can be hard too.
I understand that he has been tested for FIP, but we do not have any tests that definitively diagnose FIP. In early cases of FIP the levels of blood antibodies may be low, even though that is the cause of his symptoms. So while high levels of antibodies rising over a period of time may help point toward a diagnosis a lack of or low numbers of antibodies does not definitively rule out FIP.
If your veterinarian is at a loss and he is getting sicker (as he is) I highly recommend taking him to see a veterinary internal medicine specialist. If there is lots of fluid in his chest they may be able to visualize things better via an ultrasound and if a mass is seen a fine needle aspirate could give you a more definitive diagnosis.
Best of luck with Fritz, please let me know if you have any further questions.