Aphasia could be caused by a stroke, and in that case isn't necessarily progressive because its the symptom of brain injury not chronic degeneration.
So I wouldn't say its beyond question that he could improve somewhat given time and therapy.
Gotta say as well, that elderly people with cognitive impairment can be in denial about the extent of their issues. So I wouldn't assume exploitation out of hand. He might just want to work in order to maintain his sense of self-worth.
I had temporary aphasia from a stroke in 2017. It REALLY is a bizarre, disorienting experience. I was typing and speaking utter bullshit that was only tangentially related to the words my brain was TRYING to pick out. Things like "drugs," instead of "medication," and "cow," instead of "burger," and things of that nature. As I recall, it took me a couple of hours to even realize that my speech was coming out incorrectly, and beforehand, I was just completely panicked and confused because no one understood what was, in my mind, perfectly clear speech or legible text.
Even after I realized that I was messing up, it just became super frustrating, because in your head, the words are correct, and associated with the right ideas and objects, it's literally just lost in translation. Thank God, it cleared up within a couple of months for me, but dealing with that on a permanent basis? God damn.