I can garuntee that this dog has too much time on his own outside.
The dog believes it is in charge of security and yet it knows that it does not understand all the rules of human society.
To stop the barking you simply have to be a leader to the dog.
Do not let the dog out alone, in the yard, garden or anywhere.
Insist on good doggy manners - nothing happens unless you have given a command and the dog has done what you asked-