Issues in feather development

Hi all! I have successfully hatched a brood of quail. They are five weeks old and all seem to be doing well save one. He is my runt that had to have help getting out of his shell. He overall is doing great but he is half the size of my others. This week I took them out of the brooder and placed them in a coop with a run. We are in Montana so safe to say it’s getting cold. It was about 45 degrees outside when I placed them out. By evening as I went to move them all into the coop, little Mumble was snuggled up underneath one of the others. I moved them all into the coop and came back around to check food and water inside the coop. Mumble was flat out on his stomach and clearly was on the verge of freezing to death. I was able to get him warm inside and back into the brooder. While warming him I realized while he has all of his main feathers he has absolutely no down. Any ideas on why he hasn’t feathered out completely like the rest and how can I help him develop enough to rejoin his group??! Thank you for any suggestions you may have!

Unfortunately, runts often have issue. To be honest, I think he has been lucky to make it this long. He may just need some extra time to catch up, or he may never catch up. You might try some boiled egg yolk, but I doubt that will help him at this point.

Keep it in the brooder for a bit longer, if you have another one to put with it to keep it company, do so....'runts' need special care to make it but at the same time, it may be a futile attempt.
From your pic it appears to be fully feathered out? ....so it should be able to regulate it's body temperature but being a runt, it might have other developmental issues.

Egg yolk is a great idea, maybe with some water with supplements in it. A higher protein feed may be useful, what did you feed them all? Sometimes a week of 28-30% protein can give a good boost (that's what the yolk's for).

Looks fine to be outside imo, and as an unhelpful anecdote my longest lasting quail was the runt of her batch. She's had a lot of issues but there's something about her I have not given up on her, mainly leg issues and I feel she's not socially adept. A proper dust bath hoarder!!