Would really help if you showed pics of that one with its clutch-mates for comparison. A lot of development is retarded or sped up according to genetics, feed, environment, etc. I've had chicks fully feathered and trying to mate at six weeks and others that were six months old before they even started to show gender specific traits. Some chicks I could tell were males even before they had fully hatched.
Is it a bantam? The feet are pretty small. If not a banty I'd think male but I'd need to know more of its genetic line so I would know if it's just that type. Does it stand and act like a rooster? Not that a lot of hens don't!