Wow, goodness poor little fella, glad to read he has improved greatly, I just ran across this topic. Just out of curiosity, what kind of formula are you using to feed him and what is the consistency you are mixing it. I know sometimes people feed too thin of formula and they do not get the properly nutrients. Macaw babies often get some sort of bacterial infection.
I got my B&G at 2 weeks old nearly 28 years ago from a breeder, she was sick at 2 weeks and it took her nearly 6 to 7 months to fully pull out of it with vet care along the way. She spent more time barfing up her food than keeping it in her.