Taking a step back: Why exactly can't he take these classes in person? They won't let him walk? There's scheduling problems? It seems like taking math classes in person would largely address your concerns, and I don't see what the barrier is here.
Scheduling is a huge problem across schools. But they can be made to work. I've found one teacher who gets the kids and advocates (or volunteers willingness to flex) can get it done. Transportation is another issue. I've not had a problem with my kids walking between schools, but the administration looks at me like I have two heads - so instead they take a full sized yellow bus 1/8 of mile (yes, 1 block with a track in between).
Alternatively, I know of people who have made video links between schools work.
Personally, I wouldn't skip for a scheduling/logistical problem for a subject acceleration. I would press for the barriers to be addressed. However, a mature kid needing a skip across all subjects is another issue. As for the sibling issue, we take the approach that our kids are individuals with unique needs and interests. We address the needs of each child. Period.
