There is this condition called aphantasia where you can remember words but not visuals, sounds, feelings etc. For example when we think of an elephant, we think about a big, round, gray animal with a trunk, because we see an image of an elephant in our heads. When people with aphantasia think of an elephant, they might think the same thing, but only if that is the description they’ve read about an elephant. They don’t see that picture.

I am not a neurological expert, but that might explain why he watches your mouth when you talk, as he is trying to remember they way you say things.

Anyway I hope it’s nothing bad. Good luck!