Hello, one of the moderators here reached out for me to help you (this is the first I've seen of your question and I'm sorry it seems to have gotten lost on the boards)
I'm August Abbott, Cert. Avian Specialist, owner of N.CA Parrot Rescue (Broken Birds Sanctuary) in upstate California. I specialize in macaws, especially those 'broken' by abusive or severely neglectful humans. I have both a Scarlett and a B&G showing similar 'sparse' chewed up feathers like Rio is.
You MUST be complimented on your commitment to Rio in taking to him to the vet for a check up when he acted even a little 'off'. My concern is why the vet Rx'd an antibiotic without a diagnosis. That's not even prudent in humans anymore. Antibiotics given "willy nilly" like this can build up a resistance to them which means when one really, truly needs it, it won't work. Too many antibiotics is what contributed to the evolution of "super bacteria" and the reason we're so astute to what foods we eat were fed antibiotics while it was living
This said, maybe establishing a different vet would be a good idea for future needs. It's just something to think about
Now, as for this 'choppy' appearance: It could still be an infection. Was he checked for yeast? This is a simple look at a fresh dropping under the microscope at any vet. Some will even let you rush a fresh dropping (collected on a piece of waxed paper or parchment) in for exam without having to bring Rio, but find out the procedure before endeavoring to do this
Another thing we have to consider is an allergic reaction to something. Does anyone smoke (anything) in Rio's vicinity (as in the entire house)?
Where is he in relationship to the kitchen and let's talk about what you use to cook (any coated cookware?)
Any other animals in the home?
How about his food? Describe what he eats on a daily basis. Not the name or brand, but you've got to be my eyes - what is his usual food made up of?
Finally - how are his droppings looking? Normal in amount, consistency, color? Any odor to them?
We'll get this figured out - you, me and Rio together, ok?