My Father, Who Is 90 Years Old, All Of A

My father, who is 90 years old, all of a sudden is having a very hard time speaking. It seemed to happen overnight. It is very difficult for him to get the words out. Other than having kidney issues and having to go to dialysis 3 times a week he is fairly healthy. He claims the dialysis doctor says there is nothing that can be done. Do you know what could cause this. Thank you.