My girl Lucy does this, but only during storms. How old was the dog when you adopted him? He may have been traumatized at some point by something like a storm. My Lucy lived outside for over 8 years before she came to us, and she is terrified of storms. Now even in the house she is frightened when it rains. Now I'm going to go "hippie" on you, but just bear with me. I have found an animal communicator who has talked with Lucy a couple of times now. Two nights ago there was a storm and Lucy was just fine. I know it's because this communicator sent her healing energy for when storms come. In my own opinion, this stuff is for real. That might be something to look into. They could find out what has happened to your dog that has caused him to be afraid, and they can tell him it's okay now and that there's no need to worry anymore. Just thought I'd mention it. 
-----
Kadee Sedtal
"I wish I had a dollar for every time I spent a dollar, because then, yahoo! I'd have all my money back." -Jack Handey