Baz's job was barking at anything in the air -- birds, planes, balloons, even big leaves in the fall. (Someone who runs a hot air balloon company told me that every ride had a soundtrack of dogs barking across the county.) Baz didn't seem afraid as much as indignant -- how dare you enter my airspace?
I think most animals, including humans, have at least something of an instinct to be careful of threats from overhead -- maybe your boy's has been heightened somehow. I hope you don't have a lot of geese in your area.