Penwiper seems a little ill.
Yesterday morning (Saturday), P.W. didn't finish his breakfast, a fact which was partially distracted from by Oscar also being fickle about his.
However, by dinner the difference was obvious -- Oscar ate at a normal pace, and Penwiper only had ~ 2 grams out of 19. Penwiper was, in retrospect, also quiet all day and into the evening.
This morning he came downstairs under his own power, had a little breakfast (~4 grams), drank a little, and used the litter box, so he is functioning. Since then he has been up and down the stairs and climbed a tall cat tower to nap, so he has some energy, just not much.
We'll keep the usual eye on our catten and call the vet if he doesn't improve.
Update: Just before noon Penwiper followed me downstairs, and we were all alone so I got his breakfast back out. He ate another ~ 10 grams, much to our delight.
Of course, the other boys all sensed food may be in the air and showed up, leading to P.W. eating in the middle of the living room while Katherine and I played zone defense around him. The others were reasonably polite, and all were rewarded with kitty treats after the main course.