That whisper thing is likely autism. He's probably fine, and he's probably done it most of his life.

Maybe he’s the grown up Brick Heck, from “The Middle”.

My grandfather has dementia and this is not dementia. He might have a learning or memory issue where repetition helps process the information to make sure he understands it or remembers it. But best part? Doctors exist.

That has absolutely nothing to do with his mental state or health. It also does not prove any lack of intelligence. People on the autism spectrum and people with working memory issues tend to do that. It’s just how the brain processes for some people. Also, anxiety. I read and re-read simple sentences before I send them because I have severe social anxiety. I get scared of wording incorrectly. There are a bunch of reasons as to why he could do this.

That sounds like echolalia, which is fairly common in autism and tourettes. I would only be worried about it if it has suddenly started out of the blue. https://en.m.wikipedia.org/wiki/Echolalia

If it's a new development, it might be a good idea to see a doctor to check there is nothing wrong which can worsen over time. It must be pretty nerve-racking to have to make a conscious effort to understand every sentence, especially when you are trying to follow a conversation.

Early onset dementia is a thing but would only be considered if he recently developed these behaviours, Sudden changes in cognitive function shouldn't be ignored.

Might be OCD?

If this is a new thing, it might be worth having a doctor evaluate him. That could be a symptom of many things besides dementia. If it's not new, it's just the way he is and there's nothing to worry about.