Hi. First off, these are my thoughts and comments. Not professional opinion, and I didn't stay at a Holiday Inn last nite either. There is enormous experience with parrots on here ( not me), but like any opinion or advice , you must weigh it with the balance of your own gut feelings and evidence. I feel very bad about Buddy and in the end we all would like to hear he is better and acting normal.
Weight. As you probably know now, parrots are good at hiding symptoms. Out in the wild, signs of weakness get you singled out from the flock. Hope you've been keeping an almost daily record of his weight, since you observed any symptoms . What is it now, what has been the general trend, any change up or down? At best you can relate this to the changes being made in treatments.
Again, opinion. Amazons live in South America, so their access to all sorts of crazy supplements from China or where ever, is limited. Advice to give him naturally occurring foods, in what ever form he can eat easily, seems good advice. At least as a base line.
As an aside, I would not give Chinese made anything to my parrot. When the market there has weekly food scandals about BABY FOOD AND formula being adulterated and faked, and many things like that, doing the same to a pet food or medicine where there is little to no supervison seems all to real. I know this, I am over the 2 to 3 times a year and if I could I would bring my food with me for the entire length of my stay.
More opinion. Work with an avian certified vet. No horse Dr's etc. Ask to see the certificate , should be fairly recent too. Certified in 1965 is not the same as 3 years ago. Your in S. California? Should be many of those in and around the area. Don't be afraid to question the vet about anything, same as your own prognosis and plan of treatment. Too many folks take Dr's word as God's words. Do your research (sounds like you have) as best possible and if their plan seems odd etc, question it. Get fellow owners in your area for Dr recommendations.
Heck I'll give you my vets contact here on Long Island NY if you need it. She is one of the good ones.
A good vet will sometimes set up a payment plan, if the test or treatment is prohibitively high. They know what tests and surgery costs. Ask about their past history with similar cases. Are they spit wadding with the diagnosis and treatment or working from successful results.
More opinion. When a living thing gets sick, people, birds, horses , etc, carefully look at the environment is important. Me, I would have buddy in his cage, on my person or on a clean stand or play area. No walking about on the floor, since your shoes drag in every kind of crap from the streets, one of the worst disease vectors possible. Cage.. clean, as in really clean, best wouId be steam cleaned and often too. Same for play stands and as often as you can do it. Personal interfacing .. no makeup, lipstick, fragrances, hair dyes, nail polishes and anything else that is not YOU skipped until Buddy is well. Hands washed well and anti-bacterial used before handling him.
Opinion, the last. X-rays seem indicated. Barium enemas perhaps, but like I said question everything. Keep in mind Buddy's stress levels to continual testing and treatments. Last year, a macaw died right in the Dr's office while having blood drawn for the umpteenth time ( I asked the Dr what had happened and she was honest and upfront about what had happened).
We all here want to see the best results for poor Buddy's health and recovery. Are all the answers here or other parrot forums, etc? Likely no, but I have found some good advice and superior support from a wide spectrum of people who have the same disease we all have....
we love our parrots. Keep sane in all this, and I wish you and Buddy well. Blessings on you.