Two things popped into my head reading your post, although I don't know how feasible or appropriate they would be:

1. Can you talk to the head of the maths department at a local university and explain your DS's position to them and see what they might suggest? Maybe they have a solution that you haven't thought of.

2. Is it possible for your DS to take a break from maths at school but instead have a private tutor working with him in more obscure maths or with math applications in other subjects such as science or technology?
She thought she could, so she did.