If he is still having trouble after a course of antibiotics, I would recommend a "culture and sensitivity" test to determine what antibiotic would work best.
I have nothing to add but my thoughts and prayers are with you and your little guy. My heart breaks for both of you. I'm glad you reached out to this forum because if the answer is anywhere it is here, and of course your aviary vet.
Just to provide an update, after his latest vet visit the vet determined that the stool culture showed a form of bacteria known to be resistant to regular antibiotics. He set us up with a 2 part antibiotic to be fed 2x a day, and we started giving him that medicine in combination with the vitamin feedings. After a couple bad days, including one where my boy outright threw up some form of yellow chunky garbage, he has shown massive improvement!
He is now eating a lot more, poop is looking better, he is more social and more active. He still has another week to go on medicine but he looks and feels a lot more solid and stronger, and more alert. I really think the next vet visit will finally have some good news. i will provide an update when i get one!