If he is only a cm long in body then he should have been eaten by the other fish. What colour is he? If he is coloured and not the baby colour then the fish is seriously stunted.
Can you get a pic of him wth the other fish?
If he cant eat then it could be a stone thats stuck but his size doesnt suggest that, another is a mouth tumor, or he is too small to eat flake yet.
Try something soft like a bit of mushed boiled pea or a bloodworm or two. Brine shrimp as well might get his appetite back.
Is he big enough for you to lift out and see in his mouth?