Hi Rayopina,
       First of all let me say it's great that you're taking action and getting involved with your son's academic career. I'm sorry to hear about your current issue, and know that while teachers and educational staff are some of the most important people in our society, many can be disappointing for students.
   Fortunately, there are many options on action here. Depending on what type of school you are in, the district, and the state, many factors can effect your course of action. If you are in a public school I would advise contacting your local school district office or school board and see your options. Also, parents can have quite a bit of power in school systems, so also look into support from your schools PTA.

 here are some articles with advice on working with schools and teachers to get involved in improving your son's education:
http://www.education.com/topic/child-advocacy-at-school/
http://www.education.com/topic/parent-teacher-communication/

Also, depending on your location and economic situation your son may be entitled to funding for supplemental education:
http://www.education.com/reference/article/Ref_Choice_Supplemental/

Hopefully these are helpful, but if not please let me know what else I can do to be more specific!

I'm really sorry to hear about your son's situation.  You are being a great parent by 1) being concerned, 2) taking action by ensuing he gets help, and 3) for caring about his well being and school progress.  
I think the best way to approach this subject is to speak to the teacher and principal together.  Show them that you have taken all necessary steps, and your son's grade has improved.  Also, you may want to mention that you will continue ensuring he receives the math help he needs.  In addition, holding a student back one grade affects the child socially and decreases his self-esteem.  I'm sure that the teacher and principal wouldn't want him to feel less than.  You may want to mention that he has been crying excessively, and holding him back could traumatize him both emotionally, socially and academically.  I believe by making your case clear,hopefully the school staff will make the correct decision.
I wish you and your son the best of luck,
Rachna

The following link is helpful in understanding how to communicate with a teacher.
http://www.education.com/magazine/article/Navigating_the_Parent_and/
Best of luck!
Rachna

Check carefully what your state's criteria for academic promotion are. (SEE YOUR STATE'S DEPT OF EDUCATION WEBSITE) In the event that the school is insisting retaining your son, you can contest the promotion if you are well-informed. It is your right to see evidence that your son has failed to meet these criteria. You can ask to see a portfolio of his work and the "rubric" (grading criteria) for the assignments. It also helps if you use educational language to show you have done your research. When you are speaking about the school's failure to do anything to provide extra help until the end of the school year, you can say "the school failed to provide any ACADEMIC INTERVENTION SERVICES or an ACADEMIC INTERVENTION PLAN". These two terms can make administration nervous, because schools are responsible for investigating academic delays.