Maisy, one of the local universities has a huge and well-known vet program and they would have been willing to perform an autopsy IF A) it hadn't been a 3 day weekend and B) we hadn't put him in the freezer. Apparently freezing is actually the worst thing to do because the cells are destroyed upon thawing.
So, because we froze him, no autopsy could be performed and he was laid to rest yesterday under the memorial rose bush for my late aunt. I guess I'll never know what killed him. I did find that somehow one of his heaters had broken but everything was contained inside the heater guard and the heater hadn't been plugged in since June.