Every time I look in the App Store there is another app designed for this population. The most exciting recent discovery is the wave of apps that support scanning and single switch use.
It offers communication boards with pictures, symbols, and words. In the Settings it allows you to customize how the student will interact with the app. You can turn on scanning, step scanning, auditory scanning, define how distinclty you want the selected cell boarders to be highlighted, and the scanning speed, to name a few.
You can use a single finger anywhere on the screen with little movement or pressure to start the scanning. It can go cell by cell, or row by row.
Pairing this with the bluetooth iPad Cordless Super-Switch from R. J. Cooper makes this accessible to all users.