There actually is a PLS forum within this group here (go back to the main page where all the forums are listed)
PLS stands for Primary Lateral Sclerosis.
The following is from a Johns Hopkins webpage:
Like ALS, PLS is a disease of motor neurons. It’s progressive and causes nerve degeneration. But unlike ALS, PLS affects primarily upper motor neurons—those whose nerve cell bodies are in the brain and which deliver impulses to, and thus control, the activity of lower motor neurons. The latter innervate the muscles of the face, throat, larynx and limbs, the trunk and respiratory muscles. With ALS, there’s degeneration of both upper and lower motor neurons.
The disease progresses gradually—usually over 20 years—and, though life-changing and disabling, it isn’t fatal
PLS may also resemble one or two other diseases early on, such as progressive multifocal leukoencephalopathy, but they’re fairly quickly crossed out because of faster progression.
I do know that some people start with bulbar onset of PLS, I am not very familiar with the statistics etc, for this condition, as in my personal experience, the first hint of MND was an abnormal EMG, so PLS was not explored really.
If I can find the information about swallowing versus speech problems presenting first, I'll post it for you.
Be encouraged that his doctors think he still has many possible causes, as frustrating as it is to not know, there is always hope they will find a treatable cause.
If he has not had a modified barium swallow test performed by a speech pathologist, I'd think this is something that should be done in the interim while he waits for diagnosis, to get a feel for how severely his swallow is compromised.