I, too, have only seen this work well when it is planned for a group of kids. For instance, my older dd's middle school started a program her last year there where they offered incoming 6th graders who had advanced achievement scores in math the option of taking a compacted math class that covered both 6th & 7th grade math in one year. The 6th & 7th grade math classes were both, essentially, a pre-algebra class and could easily be combined. Those kids could then move into algebra in 7th.
Trying to make it happen for one child in a class is probably going to be hard for the teacher and, therefore, happening with a lot less actual instruction for the student. Is there any possibility of subject acceleration if compacting isn't something that works well?
We found the first year of math subject acceleration a bit painful with some missing concepts for dd11 but it has gotten easier in year two. I think that, when subject acceleration (especially in math) happens later, it can be a bit more of a challenge b/c there are more missing pieces. If your ds is really far ahead, though, there might not be much missing at all.
