My Bubba is dying from PDD. He's a Severe MaCaw.

[hlthe2b]

1. Oh, how devastating. I am so very very sorry, L0oniX

He should have been with you for many many more years.

I am sure your veterinarian is aware that some success has been achieved using high doses of anti-inflammatory drugs (e.g., steroids)
Not trying to second guess them, but infectious disease epidemiology is my field and I was vaguely aware of this newly discovered avian bornavirus

Can PDD be Treated? http://www.veterinarypartner.com/Content.plx?P=A&C=10&A=3134&S=0

For many years PDD was considered terminal and the only treatment provided was supportive care. Easily digestible food was provided so as to maximize the amount of nutrients that could be absorbed by the sick bird. Supportive care also involved treating secondary GI infections and maintaining a high level of hygiene to try to prevent secondary infections.

In the past several years, more and more avian veterinarians have reported success in the treatment of PDD by using high doses of powerful anti-inflammatory drugs. These drugs are thought to work by reducing the amount of inflammatory nerve damage that occurs as PDD progresses. Several birds with PDD confirmed by tissue biopsies have survived many months and in some cases several years with this treatment. While this treatment option holds hope for many birds, keep in mind that no scientific studies have confirmed how useful this treatment is, and as such success is considered anecdotal as it is not proven scientifically.