I personally make these mistakes as well, sometimes I try to rush through a word or a sentence. I am mostly fine, but it still happens to me sometimes especially if I try to talk quickly or I am nervous. Sometimes I almost freeze up, and am unable to say a word, when I do that as well. I really don’t think it has got anything to do with his mental state. And if I am being honest, my stuttering as a kid, resulted in a lot of people calling me r****d etc, it was actually quite horrible. And I guess dementia is the old people version of r*****