Actually, I did something similar with my #1, for basically the same reason. We chose not to unschool, but to homeschool. I was seeing a child who loved school and learning becoming burnt out in the effort to finish every single piece of paper assigned, no matter how pointless, leaving no time or energy for any of the other things that normally brought joy and inspiration.
Socially, we heard some wistful remarks in the first year of homeschooling, but net, we saw a different, much happier, and more relaxed, child. And it's not like there were no friends who stayed in touch, or new friends made through the homeschooling community.
I know others who have gone through a similar process (not always for exactly the same reasons), who often had to allow for some deschooling time, to lose the stress of the b&m school environment. You could always pull him out for deschooling for a few months of eighth grade, and then homeschool for the second half of the year, and re-examine ninth-grade placement near the end of that year. It doesn't have to be either/or.
