I would echo pb on thoughtfully weighing the pros/cons of opting out. If you are in one of the many PARCC or SBAC states, there are numerous embedded accommodations that are accessible to all students, can be pre-selected by the principal's designee, or can be activated via an IEP/504. These include wordprocessing supports, calculator, read-aloud/text-to-speech, individual testing (w/or w/out a familiar test administrator), noise buffers, item masking, adjustable color/contrast, and limited glossaries. A smaller selection is available for paper-based testing. If he is to use accommodations on future high-stakes testing, you might want to consider allowing him to use state testing to practice and become comfortable with them in an on-demand setting.
I also suspect that, by the time my one for whom this might be relevant reaches them, the SAT/ACT may look quite different, especially from an accessibility standpoint.
