I am so sorry your cat is so ill.
Unfortunately most of the disorders with this symptom are quite serious.
The worst being FIP:
However,there are bacterial infections and injuries that can cause this and be treatable.
The best ways to proceed:
drain the fluid
script lasix or other diuretic
antibiotics like clavamox or clindamycin
test for coronavirus
Once the fluid is drained your baby may feel more like eating. If he does, I recommend something very mild like baby food.
I think your vet may have already done some testing and is on the right track. However, a second opinion via a feline only vet is always an excellent option.
Please let me know how your baby is doing I will always be here for both of you.
Sincerest best wishes,
Thanks for your accept.
One of my colleagues wants your vet to check for: Chylothorax
Sending as an info request so you are not charged again.
Best wishes and prayers,
I am so very sorry for your loss. My heart goes out to you.
You made the right decision because you loved him and you did not want him to suffer.
some day you will see him again:
My sincerest sympathy,