toby's daddy here: we had to put toby to sleep today.
He was suffering from acute kidney failure; his creatinine levels were off the charts, well past the Stage 4 thresholds.
He had been losing weight over the last couple of years, despite a deliberate diet; he was skin and bones, but he was happy and bouncy until a couple of days ago. He may have had a stroke; cats can recover from strokes, but not so much from kidneys that damaged.
I may post some more photos... but thank you, and goodbye.