It really is a tough one. I wonder if you can walk through his reasoning process with him. Maybe talk with him and the neighbors at the same time.
He might say that they are too tall and he's afraid. He's probably thinking about the trunk and the branches, not the entire tree. He's likely thinking of the physics of the above ground part of the tree.
Sometimes when I point to a tree when talking about tree health, I specifically point to the ground far away from the trunk and say "your tree there" meaning the roots. People forget trees have roots. I explain about critical root zone dimensions, and try to talk about the biology.
I try to say things like "you can surely get rid of the infection in your hand by ripping your arm off, or maybe a cleaner amputation. That's about what happens with topping." Using "proper pruning and tree care" and "amputation" in the same conversation helps people to distinguish, IMO. They can surely shudder to think of that solution. It gets them to think of biology more than physics alone.
Literature about not topping trees is great, but pictures of root decay and failed topped trees can go a long way, too. I'll tell them about people whose topped trees survived the big storm, like most other trees, as few actually fail in the storm overall, only to have the root rot out and the tree ending up on their house 5-10 years later, after spending a bunch of money on the topping, and re-topping. Then I'll talk about hazard mitigations and good cultural practices.
I'd ask them where is the verified information that topping is a good practice, if they are really adamant about it.
I might even sometimes falsely say that I would top their tree, followed up by the removal price of the topped tree with its dangerous, typically included bark re-sprouted tops, and the guess-tamite of how much it would cost to remove it from their house (deductible, plus clean-up after its off the house and the insurance company pays $500 toward disposal) after root or stem rot occurs. Then, I can mention their insurance rates, and that the insurance company will then be nosing, possibly requiring them, out of their pocket, to remove other trees.
When I explain to people that I could have my ISA-CA credentials stripped for topping trees because it is malpractice and sooo damaging, they seem to hear that pretty well.
If all that doesn't work, nothing will, IMO.