Day 1 of oral GS with dry FIP?

Good evening all,
Our boy Izzy has had significant weight loss the last couple of weeks, he is down to about 8 pounds from 15. He's very lethargic, very wobbly/shaky, and refuses to eat. He hasn't had a bowel movement in days. He seems almost confused while he is walking around and just stares off at things sometimes. His water intake is normal.

So far, the vet has found he has a very high globulin level. Testing his protein levels, they found 2 out of 3 of them were elevated. They noticed some gas build up in his intestines that has not increased, as well as enlarged kidneys. X-rays didn't reveal anything else. They prescribed us .4ml of Elura to help him eat, which half of the days, it works, and he takes a few bites of dry food as well as a few treats. Other days, he does nothing. He also has a weird reaction sometimes to where he gags shortly after the med and just lays down.

Today they were able to rule out multiple myeloma and narrowed it down to dry FIP or lymphoma. We wanted to get started on GS ASAP, so she prescribed us the oral tablets and we picked it up immediately. He is currently on 50mg of the tablets, and he takes 1.5 of them daily. Today is day 1.

My biggest concerns are his eating; even with the appetite stimulant, he doesn't eat much if at all. What can we do? And with how it's progressed, does he have a decent prognosis? Thank you so much in advance.