I'm no expert, but here's a trusted link.
Proventricular dilatation disease (PDD) is a very serious and often deadly disease affecting a wide variety of parrot species. Symptoms often involve one of the following conditions; vomiting, weight loss, passing undigested seeds in the stool or show neurologic signs. Avian Bornavirus infection...
vcahospitals.com
Also...
Certified Avian Vets
Find a Specialist ABVP Diplomates have proven knowledge and expertise above and beyond what is required to practice veterinary medicine. Beyond that, they have earned the privilege to specialize in the treatment of one or more categories of animals. Diplomates are in 49 of the 50 US states...
abvp.com
If none are near you...
Avian Veterinarians
In my opinion, any of the vets listed here should be better than a regular vet.
International contacts, too.
If none are near you, maybe you could call and ask for a recommendation for somebody in your area.
And... sometimes, distant vets will offer brief thoughts or advice...
Or... sometimes I find a place to start just by Googling "avian veterinarian near ((your location))"
Or maybe a local breeder might help... again, maybe Google?