The person who owns the DAS must ride, so your DS would have to ride to use the DAS. If he is the one who is scared, you will not be able to use the DAS.
It does sometimes seem to depend on the CM at any given ride whether they'll allow Rider Swap for a child who is tall enough to ride yet chooses not to ride. If it's an important ride to the rest of your family, my suggestion is to use FP+; schedule one for each person in your party (including the non-rider), 1 child can get an extra ride by using the FP+ from the child who chooses not to ride.
Enjoy your vacation!