How old is your bird?
How bad is the pneumonia?
Aspiration pneumonia or something else?
What treatment is he getting?
"Pneumonia
Cause and Treatment: Pneumonia is actually a condition with a variety of causes, including infections of viral, fungal, and/or bacterial organisms. In addition, pneumonia can result from aspiration (inhalation) of foreign material into the air sacs. This last can occur during hand feeding, administration of medicine or anesthesia (if the bird has not been properly fasted prior to the procedure.)
One cause of bacterial pneumonia, Pasteurella, was discussed previously. A common cause of fungal pneumonaia is Aspergillus. A well-known cause of viral pneumonia, Newcastle’s Disease, comes from the Paramyxovirus groups (see below), at least four of which can infect parrot species with varying degrees of susceptibility."
So, there are a lot of variables that affect prognosis.
The age of the bird. (Young chicks with aspiration pneumonia have a very high mortality rate.)
How strong he is.
The type of bacteria/disease process causing it.
How early treatment begins.
How well he responds to medication.
Whether or not there are secondary infections.
These are things to discuss with your vet.
Yes, some birds do fully recover.
Others do not. It can be fatal.