I just got back the initial results of his necroscopy - no cause has been found yet. The suspected stones on the xray turned out to be small mucus bubbles around tiny calcium deposits, his bladder had no signs of inflammation. His gall bladder (which the vet said is often filled with bile in piggies that stop eating) was pretty empty. No signs of inflammation anywhere. Tissue samples have been sent off for more details.
Now it's just Smudge and me. In the coming days I'll need to decide whether or not to neuter him in preparation for finding him a girlfriend, but it breaks my heart to think about it right away.