Thank you @Keit for your reply! My vet prescribed sublingual Buprenorphine, which is quite difficult to administer . I will try to suggest to give an antispasmodic.Very sorry to hear about your cat.
It's true that kitties often won't eat if the pancreas is inflamed or if there are problems with the liver. Pancreatitis is also very painful. Don't know what kind of painkillers the cat was given, but ask your vet about spasmolytics (antispasmodics), like Drotaverine. Many painkillers are NSAIDs, and although they can be helpful, they can also cause damage. But your vet may not be using antispasmodics like this in his practice. Don't know.
Personal observation is that many cats won't eat if the are in pain, and often antispasmodics help.
Also, better not to give tuna or any fish, but try giving cooked chicken. The muscle, not the skin.
Also look if you can buy pure rose hip syrup. Don't worry about the sugar content. It is high in vit c and other beneficial elements, and will assist with excreting bile and maintaining liver health.
Is sodium ascorbat good for the cats? I will try to find the the rosehip syrup anyway.
About food,he doesn't want to eat anything else but a little tuna(he eats a very small quantity). I tried to give him chicken and he was not interested. Maybe when he'll get better I will try to switch to raw food, as I saw a recipe on this forum.
Thank you so much for the advice!!