I assume that you are in a public school system. In ours, both the district and the school has a website that details the grading system and the scoring rubrics. So, if you dig around in the school's website, you will find how the grading works. Or email the teacher or call the school front office and they will be able to give you the information.
As for what your rights are, you should get in touch with the teacher and let her know that your son's performance in math concerns you and that you would like a meeting with her to see how to improve his math proficiency. In the meeting, explain your concerns and ask for the action plan on how to get more math instruction for your son.
As an aside, please check out Miquon math - it is an excellent resource for that age group. Also the free math program MEP:
http://www.cimt.plymouth.ac.uk/projects/mepres/primary/You can use these to after school math.
Good luck.
