I am sorry you are going through this. How many times has he been tested?--the feather tests aren't accurate at all, so blood is the way to go (even though it still produces false negatives due to the nature of this infection.
EDIT-
Okay, so I wrote my first reply about PBFD, because I spaced and mixed the 2 up. PDD is not very well understood, but certain strains are thought to stem from ABV. Your other bird is likely a carrier and this means that he can spread it to other birds without showing symptoms. It is also very hard to test for, as false negatives occur frequently. In carriers or even birds who have just been exposed for the first time, the virus can sometimes flare up many years later (as few as 1 month and as many as 10 years). It has confusing incubation periods and is fairly contagious...That having been said, they don't fully understand why some birds are more at-risk than others. One theory actually posits that previous exposure to certain strains can increase the likelihood that a bird will contract the illness when exposed to subsequent strains. Thankfully, this one doesn't live as long on surfaces (don't get me wrong, it can live for up to 6 months-- but PBFD can be viable for years). It can still be spread through things like feather dust on your clothing and in your hair to other birds...so, keep that in mind when handling other birds or visiting places where birds are housed.
How is your humidity level in the house?
Do you use any household cleaners, such as bleach, windex, lysol, pinesol, scrubbing bubbles, kaboom, comet etc? Any cleaner is going to be unsafe to use in the same home as a bird unless it is something like vinegar and water (unheated), baking soda, an avian-safe veterinary cleaner (like f10 SC), grapefruit seed extract with water or some varieties of natural cleaners..
Do you use any air fresheners, candles, or scented oils? These are also bad for birds.
Also, he could have aspergillosis (this fungal infection is tough to catch because, in some birds who have had it long enough, the body adjusts and white-blood cell count will be normal). It takes a combination of x-rays, blood, scopes, swabs etc to pin it down definitely in many cases, and treatment is not very easy even if caught..it tends to be a constant battle with flare-ups etc etc. Nebulizer treatments containing anti-fungal agents are often used in aspergillosis cases. If it is aspergillosis, you will want to keep everything very clean and keep your bird's immune system as strong as possible. If you aren't already, consider a daily avian probiotic, healthy foods (no peanuts, no sunflowers, no salt etc), and make sure that the air is free from irritants/dust etc. I have a Veve hepa purifier that I run for my cockatoo. If you get a purifier, make sure it is non-ionizing/non-ozone producing and make sure you vacuum frequently (preferably with a hepa filter vac)
