Yes, it often does feel like overnight when a non-sexable young chick turns into an obvious cockerel. Not the nicest surprise, if you're hoping for females!
How old is it now?
I do not think you made any mistakes. Some cockerels take longer than others to become obvious, and this one looks like it has a naturally small comb to make it harder. I can't decide if it is just barely showing male-specific saddle feathers (narrow pointy feathers that...