I am sorry to hear you are so worried about your cat. My name is Rebecca. I am a veterinarian and will do my best to help.
The enlarged heart, and the fluid in his lungs and stomach does suggest this could be heart disease. I know he is young, but we sometimes see cardiomyopathy in young cats (males more than females, long haired breeds more than short haired breeds). If you check his gums, are they pink in color or pale?
There are also viral diseases like feline leukemia and FIP that can cause fluid buildup. Usually we see this in young cats, but usually like kittens to 2 years of age. Do you know if he was tested for feline leukemia when they did blood tests?
Do you have a copy of the blood test for me to see?
If the lasix helped, I would continue him on it. You can get a pill or liquid form of lasix from his veterinarian.
The chest lavage probably means using an ultrasound machine to remove fluid from his chest. That would give a sample of the fluid to analyze, which might tell what his problem is.
Try offering tuna juice, or chicken broth to see if you can get some fluids and some nutrition into him.
I agree with the moderator; this could be serious. Seeing a specialist is probably best. If he can't travel that far, does any other vet in your area have ultrasound?