I'm sorry about your father. Must be very hard, for him and you both.
There may be apps specific for his problem, I don't know. ClaroSpeak and Cowriter US speak words as you write. CoWriter costs more, and is a little more forgiving of misspelling. Both offer word choices as you type that you can touch to select, easing the process of typing longer words. I think they are not specifically TTS exactly, but they may help. Good luck with this!