I sent you a private message (flashing envelope).
High-performing high schools may be better suited to mildly to moderately gifted students who are also high achievers. They give lots of homework, but nothing will be super-deep. This model works reasonably for a chunk of bright students, but the heavy workload can be stultifying. If the school is large, your son will likely get next to no personal attention. The smaller program may provide individualized attention, but the standards may also be low. I'm writing his as the parent of a HG+ tenth grader who has experience with both types of school in California. I think it's important to know this information so that you can make a informed decision. I suggest contacting the alternative school and talking to them at length about your son. People in alternative ed. tend to be more flexible than people at big high schools, primarily because a) their students don't fit the mold and b) they're a lot smaller.
