I am also left-handed, but I don't turn my paper to write. My letters are straight up and down, as opposed to being slanted to the right, even when I write in cursive. And my hand is held straight, my thumb being parallel with the top of the paper.
The hardest thing for me in school, was trying to get comfortable in those desks which only seemed to be made for right-handed people. It was uncomfortable, and I would have to lean over and turn in my seat in order to write on the paper, but I got used to it. I don't even know if they have those type of desks anymore. I would suggest he NOT use one, if at all possible. I had neck pain all through school.
I don't remember anyone teaching me how to write left-handed, just how to write letters. In teaching my 5 yr. old DD to write - I write the letters however I write them, and she watches how the lines and curves are formed, and in what order. I don't think it really matters which hand you write letters with, as long as he watches you and learns in what order to draw what lines, etc., then let him do it his own way.
My youngest brother, I believe, was supposed to be left-handed, but I remember my Mom always taking his rattle, or other toys, out of his left hand and putting them in his right hand. I don't think that was right, and I think he had alot of problems with writing. I think you should let kids BE what they are meant to be....left or right-handed.