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I am suspicious that your little man may have a diffuse cancer in his liver and spleen - that means there is no distinction between normal tissue and a tumor. Instead the cancer cells have infiltrated the entire organ, and we could only say this for sure by looking at a biopsy under the microscope.
I think the medications that your vet started are appropriate, along with the diet change. It is possible that the little man's problems are due to an infection or something he ate (like a mushroom in the back yard), but at his age it is more likely a chronic progressive disease like cancer.
Your vet could prescribe an appetite stimulant like mirtazapine, or prednisone (which has the side effect of appetite increase). If he does not like the L/D, you can add anything that will get him to eat. It is more important that he get nutrition than eating the prescribed diet.
I would consider giving him about 7-10 days to respond to treatment. Most dogs will either begin bouncing back right away (in the case of a mushroom toxicity for example), or will go down hill quickly.
I'm so sorry you are in this position.
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