Curly feathered button?

Ok i have a 6 week old silver/ivory?(he is solid silver in colour, no pattern, with a pinkish underside. His father is the exact same) button chick who appears to have curly feathers. Sorry i cant upload a pic.

Im wondering if he is a true Curly or if he is suffering from a lack of something?
All my birds get gamebird starter crumble, with some seed mixed in once a week, plus treats like broccoli, boiled egg, every few days.
I have noticed he seems to be a slower developer as he still has head fluff and still quite small.

I normally keep them outside as we have a good climate for them here, but have been keeping them in the shed alot lately as we've been having Alot of rain and flooding, so that may be a factor. There are windows and lights on during daylight hours though.

So what do you guys reckon?
Am i missing something?
And does anyone have pics they can show of their Curlies?