Look at it with a telescope to see what it is.
Ask him if that is a camera.
It’s not unusual to have surveillance cameras outside of homes, especially vacation homes.
Put a spotlight on your house pointed directly at it.
Sounds like a piece of advice from Sheldon Cooper (of "The Big Bang Theory"):
"I always use a small mirror mounted on a stick to determine who is sitting in the bathroom stall next to mine. After all, there are all kinds of weirdos in the world!"
And I always tape over the little red light when I am "monitoring" my neighbors.
Regards,
It’s a very safe neighborhood. I don’t have any problem with him directing a camera toward the street or the river but do have a problem pointing one across the 10 feet of his property to the property line and into our house.