It appears to me that the schools have been willing to provide all kinds of appropriate assistance for your son. All of those things -- coming in before and after school and a special LD class -- are directions that I believe would have been helpful to him.
At this point, I urge you to accept the teacher's coming into class to assist him in his math work, even if it is just for awhile. Then, talk with your son. What does he want? What is he interested in doing? Then go with him to the school counselor and talk about options: Are there alternative schools in your area that could help him? Would it be best that he study for his GED? Are there some summer work-study programs that would give him a chance to learn and earn.
Whatever program or plan is developed, trust that your son can do it. Don't reject it because you think it will hurt his self-esteem. He needs to take responsibility for his own behavior, his own learning. He needs that opportunity to try, to fail, to succeed.