My dad had Parkinsons. During the last stages, in his late 80's, he was pretty much vegetative, due largely to the drugs they give sometimes, and these also impaired his cognitive abilities. During the earlier stages taking him out on foot and by car was no problem at all, he was just a little frail and shaky, there were no sudden uncontrolled movements that would have caused alarm or danger on board an aircraft.
Some Parkinson's sufferers can become slightly aggressive or nervous in the presence of people they don't know.
Not knowing the particular person it's hard to say more but I would not imagine you will have a problem and he will probably appreciate the ride enormously.