Both my children have been taught piano by an 84-year-old in our village. He is sweet but not exactly in tune with modern methods of teaching. Eventually, after a series of very mediocre exam results for my son, we moved him to another teacher. First exam after he moved my son got a good merit in a jazz piano exam (I know exams aren't everything but they are a useful monitor of progress).
My daughter is still with the old gent as she likes him better than my son does. She's just a got a really scrappy pass at Grade 3. I asked the teacher to give her a mock exam and he marked her as being of merit level--possibly even distinction. Previous marks have been a near-miss on a merit at Grade 2 (with me doing a lot of extra coaching at home, which, frankly I just can't do any more as I'm so busy); and a scrappy pass at Grade 1.
Obviously something isn't working. I'd like to move her to my son's teacher, who is young, has children the same age and just understands how to motivate them and make it fun. But--the old gent. has become a bit of a family friend and is very sweet. I also think he finds the money useful as he's not well off.
SO tell me what I say to him. Usually I'd be honest but given his age and vulnerability (he's widowed) I can't bring myself to say that I just don't think he can do the job any more.