My left handed mom, sister, and step-daughter all use the mouse with their right hand. However, I have a co-worker who uses the mouse with her left hand. Odd thing is, though, she does not switch the "main clicker" button to be the pointer finger of her left hand (the right hand button). She uses her middle finger on the left hand side of the mouse as the main button.
I think it has to be whatever the person is most comfortable with. Is he a lefty who uses his right hand for a lot of other things? At a young age maybe he can be taught to use the mouse on the right hand side. You'll just have to work with him on it.