[Cody]

That looks nasty, do you know how long the poop has been piling up? It is important to get it removed, if there is too much of it it is possible that it could lead to blocking the passage of additional droppings. Have you been able to remove it by wetting it? Sometimes when they are sleeping, a dropping may get stuck to the feathers around the vent and block the ability of additional droppings to fall away as they are produced, so they get piled up and hardened, it can also be caused for other reasons, please take a look at this Pasting of the Vent

 

[Cody]

Is the remaining stuck to just the feathers or is it on the skin around the vent?

 

[Cody]

Is it an avian vet or one that has experience with birds? Make sure that the vet does a fecal gram stain test, that will show if there is a bacterial infection, a fungal issue or parasitic, without that test you cannot get a definitive diagnosis.
Happy Holidays to you as well, please keep us updated on his progress.