So the long story short is that we're going to do nothing. I may try some other foods to help him put some weight back on -- the doctor suggested meat baby foods, or (urg!) cooking him actual from-the-butcher-shop liver (I don't think I could stomach that). I know when Stoney was losing weight at the end I had good success with giving her whole milk. Unfortunately, milk increases mucus production, and Opie's tumor is in his sinus. Long story short: the best thing I know to give him for his thyroid and kidneys would be the worst thing to give him for his nose and sinuses.
So we all just keep plugging along. Meh.