To the young man whos grandfather is aspirating food and liquids your grandfather might need to get a tracostomy to protect his air way. My 4 year old had the same problem and we found out 5 months ago he had a tumor on his brain stem. Now he drinks thickened liquids and he is fine.Speak to his doctor about the matter more and see what he says.
The first question to be answered is, why is your father aspirating and can it be prevented? The majority of aspiration pneumonias are caused by aspiration of ingested food or gastric contents. Saliva, alone, usually doesn't trigger pneumonia. His throat and mouth bacterial flora may have changed, while in the hospital, to now include new pathogens. In that instance, his saliva could perhaps trigger pneumonia, although even that is unlikely. This may also be a situation where the first pneumonia was incompletely treated and never resolved. .
It would be helpful for his doctors to culture his nose and throat, after he is out of the hospital to guide further antibiotic therapy