Since my MIL (dementia) moved into my house, her only surviving child (my deceased husband's brother) stops by to "visit." I should be happy about that but he never calls ahead of time......just pops in at odd hours. Then he stays and stays .......talking to me - not her. If I leave the room to do something else, he follows me!! So just venting, when she moved in - her "visitors" did too and that's something I didn't even think about previously.
For the not calling in unexpectedly bit, remind him that he needs to check you're there to avoid wasted journeys, and that you need to know he's coming because you might have run out of coffee (and because it's basic good manners, but it's harder to explain that). OR, you could ask him to call ahead because you might need him to pick something up for you on his way! - Make him feel useful and actually BE useful! That would be a cool move if you can pull it off.
In a way, it's nice that he thinks he's found a friend? But I sympathise with The Curse of The Home Office - spent fifteen years as the neighbourhood agony aunt. Got some good stories, though! - and, well, deadlines, shmedlines.
You know you have to tell him to "please let me know when you are coming"! But how to say it to not offend him is the question? Just bite the bullet and do it. I'm sure you have enough to worry about daily and don't need to keep thinking about his inconsideration!! Good luck!
BIL has the right to visit his mom so I guess I'll just be waiting with my car keys beside me on the couch at all times and as soon as he appears - I'll take off to Starbucks or somewhere. Hear my resentment in this post?