How old is Charlie? Was he socialized often as a puppy? Is he a rescue? Some more information might help us give advise
Our Doberman will shy away from other dogs, always has. Even in puppy class she didn't want to play with the other puppies, she just wanted to hide behind us. It's not that she isn't social, we've been socializing her since she was a puppy and she was born into a very social environment with kids, dogs and other animals. She just is careful around things she doesn't know (unlike our yorkie, who is completely fearless).