How can a student suggest changes in grading policies
Once published, I don't see any easy way to change the grading scheme for a course "in progress." 

However, if you mean to suggest how he can more appropriately grade assignments and examinations, that's a different matter. The best way to do this would be to start the conversation with a polite inquiry into why certain decisions were made.


  I'm a little bit confused about how this assignment was graded. You told us we should do X, but you did Y instead. Is Y something we should be doing instead of or in addition to X?


This initiates a conversation, rather than looking like an attack or direct criticism. It's usually a more effective strategy for achieving results, particularly when face-to-face conversation isn't possible.