Cym,
I seem to remember learning (from some DITD parent seminar) that many top universities do not require GEDs or HS diplomas for acceptance. Maybe you could contact some admissions offices to ask.

You also may want to ensure your son gets the honors level Alg1 (if it's not too late). The high schools here do give grad credit for h. Alg1 taken in middle school, but not regular Alg1.