This must be different by state. Two of my kids attended a 6-12 school and when they took HS classes in grades 6-8, the school policy was that if they received an A, it would count in their HS transcript. As a result both of my kids entered/will enter HS with a 4.0 GPA with several classes on their transcript.
As for the college/dual enrollment question, I'm not sure I understand your question. In the inaugural dual enrollment program are they requiring him to take math classes he has already taken? I might be misunderstanding, but I don't think it matters what classes he took in MS if he is taking progressively more challenging math classes as part of the dual enrollment program. Does the dual enrollment class end by the time he's a junior? If so, he would still need to take the next level of math, and in our district, the school would pay for it. I can't imagine how your school district would not, but it's worth raising the question. My son took concurrent enrollment math classes and he did run out of classes at the community college. He could have moved to the campus of our state school, but he couldn't drive, so that wasn't an option. So instead, he self-studied with another friend and the help of the head of the math department.
Another thing to keep in mind for a mathematically talented student is that community colleges generally don't have the very best math departments and he won't necessarily want to start where he left off at community college when he starts his college career.
