Now i remember that last year i found a baby bird, then i took it to my home, what i give him was, bread mashed in water i think, yeah, then i took, like a some sort of inyecction, without the needle, then i fill it and started to fee it, what i do was hold his head that his neck was straight so the food went down to his "sack" i dont know whats the proper meaning that they have, is like a bag that is in front of their chest, when its full, that is, its no more hungry, and its actions can prove to that he is not hungry, like going back for no more.
Many things you can feed him, bread, other nutrients, you can ask for better assistance with a pet shop or a veterinarian.