Hello, I'm Dr. Kara. I have over 20 years of experience as a veterinarian and I'd like to help with your concerns about Marshall.
Certainly it would make sense that he doesn't want to go outside if he doesn't feel well and is having trouble seeing.
A loss of appetite in an older kitty can be related to chronic pancreatitis, esophageal reflux, a dietary allergy or sensitivity, inflammatory bowel disease, heartworm disease, internal organ failure, hyperthyroidism, a full or partial gastrointestinal obstruction or even infiltrative cancers such as lymphoma.
Dilated pupils can indicate vision problems. In older cats with hyperthyroidism or kidney disease secondary hypertension can lead to retinal bleeds and detachment which would be reasonable causes.
His different way of sitting may be related to electrolyte imbalances leading to muscle weakness or arthritis discomfort in an older kitty.
Ideally your fellow needs to see his veterinarian as soon as possible for an examination and some testing.
At home today to settle his stomach and hopefully improve his appetite you can give either:
1) Pepcid ac (famotidine) at ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 24 hours.
These are acid reducers and may help him feel better so that he will eat.
I see that you tried feeding small meals and soft food. Perhaps mixing soft foods with warm water or low salt broths water to make eating and swallowing more comfortable will entice him to eat. If he is refusing to eat even soft foods you can try offering meat baby foods or a bland diet of 2/3 boiled, minced, white skinless chicken and 1/3 boiled, white rice mixed with some low salt chicken broth to make it easy to lap up and swallow.
If he continues to not eat well he must see his veterinarian for an examination, blood tests, blood pressure measurement, and maybe radiographs of his abdomen and rear legs. While they are waiting for results of his tests they can start intravenous fluids, injectable antinausea drugs, an appetite stimulant and further supportive care based upon what is found.
Best of luck with your fellow, let me know if you have any further questions.