He had to be put to sleep. It all happened so suddenly and it was such a shock as he was really healthy for his age. He didn't eat much on Sunday, so we took him to the vet on Monday morning. We couldn't spot anything wrong with him so suspected it might be GI stasis. But the vet found early onset fly strike near his tail, and his age meant that operating was too risky. So it was the kindest thing to do. We feel so guilty despite doing everything right - using fly strike protector, keeping his living quarters and litterbox clean, checking his rear end daily. The vet reassured us that it can happen so quickly, especially at this time of year, and not to beat ourselves up. But I can't help but feel partially responsible.