It can be hard sometimes to know what the cause was. Anything odd about his appearance? Anything odd happened there recently, shortly before he died? That he went into the nest box suggests he felt poorly before he passed, and that it wasn't a sudden death on the roost, of heart failure for...
Sorry for your loss. Do you know the cause of his death? Can you say a bit more about his condition/ symptoms before death?
That apart, they are likely heading into moult so feeling a bit under the weather.