Duke has passed away.
It was kidney failure. I read up online a bit on it, and apparently bull terriers are prone to it. It is something that can come up as early as 2 yrs old, sometimes even as just a pup of 6 months. Medication can prolong their life a bit, but there is no cure for it. It is not simply treating a disease, it is the actual kidneys malfunction and break down and fail. Bullies are also very strong, so they don't show pain until it's very bad.
Rescue would not have been able to save this big guy. He was already too far gone once BIL even noticed anything was wrong. I was very surprised to read that the life expectancy of bullies are only about 6-8 yrs long. Duke was 6.
RIP Duke.