Are you sure that Magnet is the right place for him (as opposed to skipping into a program with a lighter load)?
We don't have a lot of other options.
There is a program that starts in a later grade that may or may not suit and is hard/probably impossible to get into. That's about it, other than private school or some kind of fringey charters.
Aha. Then I'd set about making a plan that meets DS's needs using the resources at Magnet but customizing as necessary. To wit: "because he's [young for grade, etc], XYZ isn't really developmentally appropriate for him yet... how can we accomplish this end via appropriate means...?"
It's amazing what's negotiable. Not everything is, of course... sigh...