It appears to be whatever it was that originally rendered him sick and flightless without any obvious symptoms except a bit of blood in his beak, internal bleeding of some sort - the only sign is a bit of blood in his droppings.
I feel like if I hadn't encouraged him to get some exercise and flutterscuttle around yesterday, he would not have aggravated it, and would be fine now. But he was so bored, and so happy to get outside and explore...
Sigh.