There are several text-to-speech apps available for smartphones. On my iPhone, I use Speech Assistant, which allows you to save phrases in categories and will deliver text as speech to a caller allowing him to answer or even place a call. It can be frustrating at first, but practice does make it easier.
You mentioned he is using paper and pen to communicate. Might want to consider getting him a whiteboard. I use them as I find writing faster that typing in most instances when answering questions from doctors, nurses, and other medical professionals.