The only things I can think of is
A. keep a water bottle outside and spray his face when he does it with a firm "no barking" It doesn't hurt them but they don't like it. Eventually you will just have to pick up the bottle and show it to him when it when he barks.
B. the other thing you can do is pick him up when he barks and tell him a firm "no barking" and put him in a time out room inside where he can see no one. Make sure he is quiet of barking for 10 minutes before you let him out. He does it again repeat it. It may take a lot of repetition but eventually it clicks.. |