Thanks, guys. The vet recommended layer feed to start, but we will switch to chick starter feed soon.
Watery/mushy feed has certainly been more attractive to him, but he's still not very interested. I mixed it with scrambled eggs, but the amount he eats is... pretty disappointing. I don't know what his previous owners fed him, but he's like... addicted to corn and bread. He's not even interested in vegetables, other than tomato.
The vet told me to add mushy premium puppy feed to his diet once a week. Will try that.
In the meantime, I'm kinda worried because I've never seen a crop so... small and deflated. I mean, he's a slender guy anyway (probably an Old English Game hybrid), but sometimes it looks like his crop is sinking in his chest. It looks a bit better at evening, as it's supposed to, but I really can't believe that, as a chicken, he's not into foraging and eating all day.