It seems his back might is broken poor cherub bring him out of the smashed egg (eggs are for feeding and keeping it safe the egg can't do that any more) dont put it back under the mother she will literally crush the poor baby, grab alot of feathers from the bottom of the hen house, get a shoe box make a nest out of the feathers and put it in the shoe box, get a light it can be any light as long as it produces some type of heat even a light bulb would do something, put the baby in the nest and under the light, and make this meal. Two teaspoons of crushed layers pellets, 1/2 teaspoon of hot water and a 1 teaspoon of egg white mix it all together and feed him that by opening his mouth and shoving it in he is most likely malnourished I know you want to just let the mother take care of him but just remember its mothers a stupid chicken and you have all the resources to save him
I had one of those chicks and if came around after 2 days and is still alive so good luck:/