I think that sometimes changes in lifestyle cause dogs to take up barking as a full-time hobby. Jake was never much of a barker, despite being a sheltie, but he barked a LOT more when we first got Max.

Whenever he barks I just take him somewhere else: if he barks indoors I put him out. If he barks outdoors I bring him in. So now I think he knows that if he wants to stay where he is, he'd better keep his mouth shut!

I think that if you always bring him in immediately after he starts barking, he'll learn that he has to be quiet if he wants to stay outside. They're pretty darn smart dogs. The key is consistency.