I'm sorry to hear that
Do you know where the tumor is at and how big it is?
Cause i would think that this acute reaction of your kitty vomiting and not eating is a little weird when the tumor was already there for some time but still small enough now to be sure it's a tumor?

I mean when the tumor is big enough and is pressing on his stomach or something i can understand.
But it's not clear from what i read..