Relationship Advice

Option One (Lawful Neutral):

Always be wearing headphones.

Just pretend you cannot hear him, and then if he is so annoying as to come around and see you visibly, you have a visual indicator that you are listening to a book or music or something.

Option Two (Chaotic Evil):

You now also have dementia / gaslight his dementia.

Who are you? Why are you talking to me? Don't you remember you left the stove on? Wow, look at at all the beautiful butterflies! (there are no butterflies). Don't forget about your doctor's appointment, its in 10 minutes!

Option Three (Neutral Good):

Suggest a variety of low physicality hobbies to him.

Coin collecting. Model Trains. Amateur Astronomy etc.

Ideal outcome is he manages to get absorbed into something that sticks and this results in less time being annoyed by him, and when he does talk to you it is more likely to be less random, and about said hobby. Further, if he can become attached to some hobby, you are likely bringing joy to his life and giving his wife a bit of a break.

Option Four (Absolute Chaotic Madness):

Inject him with Compound V.

In your situation, I would try giving him something that needs to go inside or something to share with his family. It's harder to stay outside chatting when you're holding something awkward. Next time he comes out, do a little polite small talk and then try offering a bowl of potato salad or some other food or dessert that's somewhat heavy or needs to go in the fridge, and you'll simultaneously be both a nice, kind neighbor and have an excuse to constantly nudge him to go back inside. "That's pretty heavy, you should get that inside", "it's been nice chatting but you should go put that in the fridge", "I bet your family would like to try that, do you want to see if your wife wants some?".

Maybe he'll go inside and then come back out again, but if it works you have an "out" that keeps things friendly. It's worth trying at least once to see if it works.

Something I'd suggest looking into is gentle redirection for dementia patients. Dementia patients respond very poorly to "no", "you're wrong", "that doesn't exist anymore" type statements. Instead there are techniques where you agree with them, make positive suggestions, and agree with their nonsense to get them to do what you want

For example: when a dementia patient starts trying to walk to the grocery store instead of saying "you can't walk to the grocery store, it's 15 miles away and you shamble like a zombie" you'd say "Oh you know what, I need something from the grocery store too! How about I go with you?" Then after they likely agree you say "Oh shoot, I forgot I'm in the middle of cooking something. Can you go back inside and I'll come back in an hour to go with you?" They forget you said that, rinse and repeat

So for this guy as he's shouting his nonsense, don't necessarily agree, but say things like "wow, I didn't know about that", "I heard that's a serious problem." Then say "I actually heard something crazier than what he said, why don't you go look into that and tell me what you think later" or if you want to turn it around on his wife you can say that she told you things and that he should go ask her for more information

Not a lot to do with dementia aside from talking with their caretakers. My FIL got on this kick that I stole his $5 Walmart sunglasses and it became pretty aggressive for a while. It kind of cycled for a while between him forgetting he was mad at me and ruining a weekly family dinner with accusations. I didnt hold it against him, its a shitty disease and hes not the man I've known for the past 15 years.

Dementia sucks. Try being direct, but politely, and tell him that you can't talk right now and hopefully they understand. Or realize when they get to this point they have about 12 months left in the tank.

What about "Hey l'm so sorry I really can't talk right now, I have to finish the yardwork before [some excuse] but I can stop by tomorrow to chat for a few minutes."

Blood capsules. A couple dozen. Pouch one in your cheek every time you go outside. Begin to cough blood every time he talks to you, then hurry back inside.

After the first incident he'll be more eager to talk to you. After it happens eight times, he'll start to avoid you.

This is received wisdom so take with a gain of salt. Dementia people respond to emotion. So if you're really nice and happy they will be too. If you're mad, even justifiably so, they just mirror that back at you.

Maybe try just base manipulation. "Oh hi neighbor, so good to see you, could you go inside please?" In just like a real friendly tone. Maybe have a snack and tell him to go eat it inside.

Wear headphones and/or pretend he doesn't exist? Make your fence taller? If his wife is still mentally sound, discuss it with her?

How are your carpentry skills? Because a fence is definitely justified.