My sweet little kitty girl was throwing up all the time, and a nice lady told me about Royal Canin food, Special #30, very good ingredients. (found it at a feed store. Since I've been feeding that to her, she has had no trouble. A little later, she did swallow a piece of ribbon, and stopped eating. I gave her some olive oil, and she threw it up.
I don't blame you for not wanting to subject Humphrey to surgery. I hope he is still feeling well.