I was so busy yesterday I completely missed this thread. That's just terrifying, and you are doing SO well to even be able to update us. I'd be jelly on the floor of the vet's office.

Have they tested him for H pylori bacteria, or any of the other related ones? That's the most common cause of GI erosion and ulcers in people, and IIRC I read somewhere that there are strains of similar bacterias that cause ulcers in cats; it's more rare in dogs, but it does happen. Just like people, sometimes animals are asymptomatic for a long time until some little thing just kicks it off.
I remember how bad my ulcers used to hurt, and they weren't ever to the point of bleeding. You're all in my prayers, especially Cowboy.