I have an impression he was entirely legendary with zero basis in history, but I welcome any explanation to the contrary. From reading about other figures on this sub, I think "attested" is the right term to go for. How well attested is Prester John and how does he compare to other historical figures?
I wrote some about the Prester John legend in this post. The popular letter purported to be from Prester John was a medieval forgery so there is no evidence he was ever a real person.