This is a kind of story I hear a lot. A student is a good student at the current Chinese high school, but somehow she decides to go abroad and to apply to some elite boarding schools. When he plans to apply, she all of sudden realizes that she lacks some serious preparations such as TOEFL, SSAT, and other stuff and that choices are limited. And she chooses to a school where she would not want to go in the beginning, but anyway have to go. And the story goes on.
My sincere advice is that he needs to plan a long term strategy, not just getting into a top boarding school. A long term goal of achieving solid education at an appropriate high school in USA while carefully planning for an elite university in two or three years. Your child should have this long term plan that make sure the student can get the best of his high school experience. Your child's application process should be based on this understanding and I am sure your child will receive benefit of this type of planning.