To address only the 100% extended time question: yes. The CB requires test-takers to stay for the entire time provided. You cannot leave early. This is the case for all of the CB's extended time accommodations (on APs, SAT, etc.).
Regarding +50% vs +100% on the SAT suite: given the conversion of the suite to adaptive digital testing, starting from the SAT and apparently moving its way down (currently at the PSAT/NMSQT), I suspect that +100% will become less critical in the future, since the low-level questions will not be presented to high-achieving students. I do find that many students are drained by the end of +100%, especially the ones who are taking multiple AP exams in the same cycle.
Does his IEP have accommodations for reduced items or assignments? Items sufficient only to demonstrate mastery of standards? Oral elaboration of written responses (complete assignment first on paper, and then orally elaborate to fill in details that didn't make it to the page)? Is he using assistive technology for written products (speech-to-text, typing), at least for first drafts?
And you can compete quite easily with honors/AP and facilities by looking into dual enrollment options at your local 2- or 4-year state colleges. Even if he doesn't place into say, college science courses that require calc, an intro science course aimed at non-science majors would still move faster than a college prep high school class. At this level, you're not looking to build a college transcript, just to keep his mind stimulated, so it doesn't matter that these might not be the level he would want on his college transcript later. (Besides, a lot of colleges will just consider them as part of the application package, not for transfer credit.)
