Hi and Welcome to the HFF
How old is Hero? Your vet may not want to do the surgery, if Hero is older and not strong enough for surgery. Unfortunately, tumors may come back and while surgery helps remove those tumors that are already there, other tumors may show up. The Surgery is tough on a little one and recuperation is rough. I think that may be his reasoning.
Does your Vet believe that Hero has Insulinoma and Lymphoma?
Hero does need to eat regularly if he has Insulinoma. We use a soupie recipe that is balanced and full of nutrients. The recipe for the soup is"
8oz of RAW chicken. Ferrets need raw, cooking removes nutrients.
1-2 chicken hearts
1/2 chicken liver 1/2 tsp of dried eggshell powder.
Save some eggshells, dry them out for a day or two and grind them into a fine powder.
Blend this up and you can freeze some of it, in ice cube trays and then just pop them out and defrost them.
After you make the soup, put some in a bowl and sit with Hero. Dab a little on his mouth and he will lick it off. Then dip a spoon into the bowl and offer it to him. He will need some coaxing and you sitting with him, until he understands that this is food and good for him.
Ferrents imprint on their food and it takes attention and patience to get them to eat a new food. Keep with it and he will get used to it and eat better. It may take a few days or so, but he will learn to like it.
Good Luck and I hope your Hero gets better.
Don't hesitate to ask questions.