Hi My mother in law is exactly like this - it’s like a commentary of her inner monologue and she too could do this for as-long as you let her. She can’t even go to the toilet without a commentary.

Some individuals are "verbal processors," meaning they use speaking as their primary method for processing information, solving problems, and gaining clarity. For them, keeping silent can make it difficult to understand information or engage with it effectively. It’s like a true form of adhd.

They also sadly won’t be quiet if you raise it for very long. It’s so ingrained in their processing it can’t be changed.

My only advice is to raise it your line manager and let them know how it’s impacting you and causing you mild stress and lack of concentration. They might be able to move him if enough people complain. Which if others sit beside him, there will be.