Poor little bird! There are so many things this could be.
Do you give your birds an anti-parasitic?
Do your quail live in the same area as chickens?
How does his crop feel?
Can you hear any clicking or strange sounds when he breathes?
With a bird in this state, he will probably die without vet care unless you are very lucky, or if you already have stuff on hand.
If you have it, start him on anticoccoid immediately.
The next thing you gotta do is poopology so you can get more answers.
While he is still able to eat, stuff him with as much scrambled egg and meal worms as he wants to get his strength back and bring him inside where it's warm and he can be monitored and kept separate.
I wish you the most luck!