I would recommend Choate... (
www.choate.edu) They have recently begun upping their amount of recruitment/yield in China, according to a news post in the school newspaper about admissions. I'm also going there next year, and if you or your relative wants to get in touch with me, let me know in a PM with their email address and I'd be happy to answer any questions.
A few more amazing schools are Exeter, Andover, Deerfield, SPS. These are off the top of my head, too.
Blair, Peddie, Mercersburg, you name it.
Another good bet you can make is to check out boardingschoolreview.com, where you can input your criteria and find schools based on that.