Is this a new behavior for him?
There are a couple of things this could be - echoalia or palilalia. Both involve repeating what other people say.
This is a call to a radio show about it - https://www.waywordradio.org/mouthing-other-peoples-words-when-talking/
It could just be mirroring, or a way of him making sure he is following what you're saying, or paying really close attention. Have you gently asked him why? Maybe just say, "Hey Dad, I've always wondered why you mouth the words when I'm talking. How does that help you when you do it?" If it's new, you might ask him if he's noticed he's doing it, along with asking him how it helps him. Don't confront, just ask.
If it's new, he might want to get checked by a doctor to make sure nothing neurological is happening.
Hello~I agree with the other answer. I am thinking it is probably normal, that he doesn't want to misunderstand what you are saying. If it bothers you, then just casually ask why he does this. I don't think there is a thing to worry about however.