Agreed, I have lost at least three mature birds that had eaten sharp objects and pierced the esophagus letting feed into the body cavity and dying.So very sorry for your loss, I know how heartbreaking it is to lose one.
If I were you I would do my own necropsy. If it was blackhead, coccidiosis, or roundworms it will be obvious. Blackhead poop is not always yellow.
Other things to look for are things he could have eaten like sharp pieces of plastic or metal, that's pretty easy to spot too.