Since you really don't know what you're dealing with, about the only thing you can do is deal with it a day at a time
Evaluate how Timber is feeling from day to day and make your decisions when they become necessary.
I was just looking for information on Imuran since I'm unfamiliar with it. Not sure what I'd do about it--but I'd for sure check with the vet and see if it's something that requires weaning.
I wish I had more advice for you, SW!