Thanks everyone, if anyone has any ideas on getting him to eat, please let me know.
He's on appetite stimulants, but will only drink. I've been finding him the most disgusting, cheap, smelly food I can find and we mix it with hot water to make "gravy" and he'll drink the liquid but won't touch the solid part of the food. For a couple days, albacore tuna with catnip sprinkled on it was working. And FreshPet salmon soft food was good for a few mouthfuls. However, he stands on the crate and yells for food, and purrs when I put it down... he just won't actually eat it. He definitely pays much more attention to "fishy" food than anything else.
The tumor has grown noticeably since last Friday. When we found it, it was two "upward bumps" between the last two ribs, now it has covered the ribs completely and you can't feel where they are at all. I can imagine that it's moving his insides around which is impacting the eating as well.
He's back to using the litter box, which is a good thing - last week he peed all over the house on anything he wanted to. The couch, the dog bed, my silk heels, the clean laundry, the dog blankets, the dirty laundry, the bagpipes...
Inside me is a thin woman trying to get out. I usually shut the bitch up with a martini.