An abdominal x-ray and ultrasound may be helpful in determining why your cat's abdomen may be bloated. I would seek a second opinion before doing anything. Abdominal distension could be an indication of another serious health condition. The common causes of abdominal distension in cats may include liver disease, heart disease, viral disease (FIP), or some type of tumor. Some of these can be treated depending on their severity. So the best thing would be to try to get a diagnosis first if you are able to before your cat's fate is decided. Other conditions would be kidney failure, but a cat with this illness also tends to vomit besides becoming weak and lethargic. I would go with another vet.
I'm sorry about your cat. I know FIP is deadly, and I honestly hope your kitty lives a happy life no matter how long he lasts.
My heart goes out to that cat. Good luck to you both.
I would definitely get a second opinion. For one thing, I always recommend this for any case that is serious where the owner is not 100% comfortable with their vet's diagnosis, communication and treatment of their cat.
In this case, there are other possibilities that could lead to fluid accumulation in the abdomen. PrettyKitty1 has listed a number of them and there are other, less common causes of blockage leading to fluid buildup, including a severe internal parasite condition. If you vet has ruled all that out, you should have at least been told, gotten all appropriate information dealing with treatment and care of your cat, including caring for him at home (like special diets, that sort of thing, not just prescribed medication). It sounds like you didn't hear all of that.
Another thing I often advise is taking your cat to an animal hospital instead of a vet's office. They have larger staff and lots of experience, so if you are near one, it's a good idea.
Good luck. Please come back if you have any questions or concerns about the situation. My thoughts are with you.