Cute quail! Sorry about his feather problem, that can be heart breaking to have a pet mutilate himself.
When does he tend to feather pick? Does he only do it when he's by himself, or even when he is with you?
Keeping him busy can help break the cycle. Wild birds have to forage to eat, they don't get a feeder full of food sitting in front of them. Offer him food or treats that he has to work for. Buy or grow your own meal worms but hide them in some way so he has to hunt for them, maybe in his cage substrate. Clip a kale leaf above his head so he has to stretch to peck at it. You can ferment his food part of the time to give him a change that is interesting, but fermenting also increases the protein content of the feed and is just different than dry. Try a few toys he might like to play with and rotate them with other toys every few days for a change. Put his cage where he can see out a window when you are not home to give him something to watch.
Try hanging a feather duster where he can play with it. Maybe that will give him enough of an outlet that he can regrow some feathers.
If you can get turkey starter at your local feed store that should give him enough protein. I'm hoping he will at least partially grow back some feathers just by the increased protein intake!