Please ditch the caps, they are annoying to read.
Quail do have muscles and if the the mass is reduced in one side, that would indicate that he hasn't used that leg as much as the other, for a while. So I don't think this has been caused by a recent accident. Doesn't seem to be caused by the feed either, as many people use that. So, what could have caused it? How carefully have you been observing them? He hasn't shown any limping or anything like that before now? He wasn't a splay leg chick?
If you haven't already, I'd give him a good feel all over, see if the muscle really is reduced, what differences he has between left and right side, clean the legs to determine whether it's both legs or just one that's hurt and how bad it is. Any of the others seem to have any issues? And how many are there?
Other than that.. Well, if it's not feed related, if the others have no issues and if my theory that it has been going worse for a while is correct, I'm afraid his chances might not be very good. My only advice is to keep him warm and comfortable and make sure he gets feed and water. Hope I'm wrong!