I don't think I missed anything vital.

I don't think the number of kids in each school matters if it doesn't matter to your DS. I'd raise the point with him, as I'm sure you've done. But if he's thought it through, then I think you respect his wishes and let him live with the consequences. (Within reason. Misery is not okay, even if it's the result of a kid's own bad decision.)

Kriston