Could just be what he ate before trap as already stated. If it was sour crop it would be unlikely that he would be eating. From what I have researched with sour crop they will just pick at the food and flick it away. If he is eating and not throwing his food back up, you should be able to rule out sour crop.
He is doing fine eating,He is flying well, getting stronger , been two weeks of the trap and is flying over a hundred yards several times.Gonna try and let it eat some casting material and see if he can "wash" it out... It is slowly getting better.
there are a couple of ways to approach this. i know you can use pepto to help with it. another way is to do as you said, get a buch of casting material, with as little protein as possible, and let it eat all the fur, feathers, to act like a cotton swab. as echo said, examine the casting. but try, for 24 hours, to just let the casting material "scrub" it out. and echo is right, quail can cause this. it is a rich food source, think of eating a pound of chocolate right before bed. try not to feed the bird really alot of really rich food to late, sounds like gremlins. also, chicken can be fed, but try not to feed them alot of chicken on hot summer nights thru the molt. could cause selmanillia (sp)? good luck with him, hope all is well.
If his cropwas full and it had an odor like that it is sour crop. Baken soda and warm water. About half a tea SSpoon in a pint of water. Use a sringe and don't try and get a lot in him, just a few ounces the first day and increse day two and three. If it's still that way in four days, then off to the vet you go. Another thing is crop worms. To get rid of them there are may different types of medications. I use a product made by Intervet Inc. it is called Fenbendazole granules 22.2 % or 222mg/g I use barley the tip of a teaspon and place the powder in the center of a piece of meat. Worm for five days and then in 18 days repeat for three days. I do that twice a year with my birds. I always worm every bird when I first catch them if I'm going to keep them.