a speech therapist might be able to help - they might be able to help with some sort of pictorial communication system that your Dad can touch different pictures or words to help him communicate?
'communicate with symbols' 'visual boards 'etc often used for those who have had a stroke
I work with children with speech difficulties and although completely different - for some having a set of relevant words or pictures that can be used to help with getting across basic information.
e.g. showing a picture of or the word 'radio' and a picture/word of a TV then he would be able to indicate which he wanted by touching it or looking at it or nodding or shaking his head......
have a set of words to express emotions, or other things where he might like to express a preference of what he wanted.... you will know some of what he might want /need to communicate about and make a set yourself .
won't say any more as I don't know your situation exactly and this might be a silly idea for helping your Dad. But would be happy to say more if you thought it might help you and your Dad.
My Mum had been heading towards a similar difficulty, and I often wished that I had taught her some Makaton signing that I use with the children at work! and I would have worked out some sort of method as I described to help her had it been necessary..........she died in 2013 aged 91