5 yr old Calico male cat has major swelling in his abdomen on the sides and the bottom.

He also has swelling on his legs and paws. His abdomen, legs, and paws are tender to the touch. He has seen two vets and had blood work, an ultrasound of his abdomen, and biopsy of his kidneys. He is also on Doxycycline and Lysics. The only thing revealed in all of the tests was a compete lack of Protein and dry FIP. Neither of the two Vets that came “highly” recommended know what to do. His primary Vet says he is having sleepless nights trying to figure this out. We’ve spent over $500 so far and we don’t have much more to spend. This website was recommended and I hope you might have a suggestion or an answer. I really don’t want to put him down since I don’t know if this is fatal or not. Can you please help?