I thought maybe people used the back door there like they do here. People almost never knock on your front door here and it still startles me when someone comes to my back door. My PC sits in the corner of the breakfast bar and my back is to the back door so it always bugs me when someone 'sneaks up on me' that way.
Apologies is a previous post has already addressed this, but didn't you say that Creepy Dude lives with his brother? His brother may be playing a caregiving role in his life, and a big part of that would be constantly explaining and reinforcing acceptable social behavior. Could you or dh maybe talk to the brother?