I'm pretty sure you're seeing slow feathering (this chick) vs. fast feathering (the other chicks.)And the others all arefeatehring out 10X quicker. I know that probably means nothing but still, I find it odd how they are starting to get tail + shoulder feathers, yet this chick has yet to get any of that? Its probably nothing though.
If the father has fast feathering (which seems likely, given what you've said before), then this chick must have a slow feathering mother, and must be a male.
(Chicks with fast feathering could be either gender, since some of the mothers probably have fast feathering. On the other hand, if the father has slow feathering, then this slow chick could be either gender.)