AGY is treatable when caught soon, it is actually a yeast and not a bacteria so that is why antibiotics are not effective, however sometimes a secondary infection is present with the AGY and an antibiotic will be used for that. The med most commonly used to my knowledge to treat an active infection of AGY is Amphotericin B. The vet can do a fecal test and see the organism in the droppings if it is being shed in them, it is not always shed so just because the organism is not seen does not mean it is not there. A bird can have the infection and not show signs, so if signs appear it can be difficult to determine how it was contracted. It can be passed from parents to chicks if the parents are infected.
There are other things that can cause a bird to pass seeds in the droppings and that is why it is important to have the bird seen by an avian vet or at least one that routinely treats birds. Do you have any exotic vets near you, they will sometimes have a vet on staff that is well versed in treating birds. Is Parker showing any signs of not feeling well, such as sleeping more, lethargic, depressed appetite, or vomiting? Keep watching the droppings to see what percentage of them show the undigested seed.