I recommend you plan FP+ for the rides his sister really wants to do, but the boy may not be able to handle.
Rules of
DAS do require that the DAS holder ride. Whether or not they may allow the rest of the party to remain in line while he exits the queue could vary. I wouldn't fully count on it to always be allowed, but you can definitely explain the situation and ask at the time. If he's finding the park(s) overly stressful and impacting his sister's experience, it may be necessary to split -- someone take the boy for a snack and downtime (either in the park or back at the resort) while another adult takes the sister for some fun; swap adults on different days.
Enjoy your vacation!