I hope it is ok that I respond for Dr. Harry- I saw that this question has been pending awhile and was unsure how quickly you needed a response.
As I'm sure you've been told, PBFD is a viral disease with unfortunately no cure. Often the feathers will be damaged first and as the disease progresses, the beak, nails, and ultimately the immune system will be negatively affected. Since you have already been seen by multiple vets, I'm sure someone has already officially diagnosed through either a PCR blood test or cheek swab?
The next step would be supportive care, which includes preventing secondary infections (because of the weakened immune system and damage to the feathers, beak, and nails), certain vitamins (your veterinary can help you determine which ones he will be deficient in), occasional fluid boluses at the vet, and making sure he recieves adequate nutrition.
Beyond that, an internal medicine or infectious disease specialist may be able to help you determine if medications to boost the immune system's white blood cell production within the bone marrow itself would help. This medication can be costly and is only used as a very last resort.
I hope this information helps in some way. Thank you for allowing me to answer Dr. Harry's question in his absence.