I am presently dealing with a dyslexic student with a very aggressive attitude towards the notion of correct spelling. This student finds spelling difficult and has taken offence to my corrections. It is not a problem, he argues, but simply a natural manifestation of his unique mental picture. He refuses to use the spell-check facility on his computer use a dictionary. As his English teacher this position is difficult for me to accept.
This aggression no doubt has something to do with past schooling. You must show some sympathy for that. I have known people with dyslexia who turned into anxious censors of their every written expression for fear of exposing their difficulty while others simply accept it, celebrating their uniqueness sometimes to the point of obduracy.
I'll guess that your student is in his thirties or forties. He was once a child of the Seventies doing CSEs (remember those?), neglected, back of the class, unable to copy from the board or concentrate during dictations; problems with spelling and worse at maths. Branded "thick", the careers lady said he was only good for the army or the checkout. Now he has a second chance and he doesn't trust you because you represent the system that failed him the first time around. So he's establishing the rules because he doesn't want to fail again, and who can blame him? The Government wants to raise skills levels, but the view that simply bringing people back into education and sitting them in a classroom for a few years will achieve this goal is simplistic. Let him know that it was school that failed him, not the other way around.
Work on his self-esteem before spelling.