so sad; I worry constantly about my littles, it's like having kids again
Maybe he just had poor immunity to whatever's endemic there...?
Check for a peck to the head if you have suspicions regarding a hen killing the chick.
I didn't see any marks on him and he was running around behaving completely normally since he hatched. But because I didn't see any marks on him doesn't mean I didn't miss something in his feathers.