Any disability, perceived or actual, with which a prospective pilot is handicapped should surely be a matter for the medical department of the CAA concerned. It should be up to the state to determine whether or not someone is dyslexic-in such a case-and then to set the guidelines under which they might hold a relevant licence or have a restriction put upon it.(Such as 2 crew' - yes, why not, even for a PPL-the man can still enjoy his flying.) So yes, the man can fly with an instructor, trying to learn to fly, but he should go through a filtering process at the time of his Student Pilot Licence medical, before he goes solo.
It is surely not the business of a flying school or an instructor to enter in to this realm other than by making suggestion to prospective pilot that he contact medical section.
Hard as it is sounding..if an illiterate wandered in to your flight school, wanting to learn to fly-you would tell him to go and learn how to read and write and then come back.