Put the question in another sport....would you take a left handed pro pitcher or hitter in baseball and try to switch him to right hand?
When I was a kid in grade school, the teachers would not allow a kid to write left handed...it created a lot of frustration and resentment on the kid.
My opinion..if he is left handed--teach him left handed, you can later develop some right hand punching technique...but work with his natural ability first.