From what you seem to be describing, this seems most ideal:
iPhone/iPad + Alternative and Augmentive Communication (AAC) app + Bluetooth Switch
For the AAC app, I would most likely recommend either ChatAble or Predictable. They are both from Therapy Box, and the features are top notch, along with having ultra-customization. There are cheaper apps (please check out this page--it is a gold mine on AAC), too.
You likely need a switch (watch the video on the page to understand the concept--but get the switch from here) since he is very sick, and he may not have the dexterity or stamina to access an iPad or iPhone with his arms and hands.
You can get an (accessible--a lot of apps are) "panic button" iOS app that will page your family or caretaker via text or phone call. He can access the app via the switch interface.
Alternatively, there are AAC books that are analog and way, way cheaper. But, they do not allow him to communicate his needs when a caretaker is outside his room.