submissive rolling over and trembling

I just adopted a dog from the pound. He's a sweet dog. But he seems to lack confidence. When he comes up to you he rolls over on his back, and when you scold him he rolls over, trimbles and sometimes urinates. Sometimes he even cringes like he is expecting to get hit.. How do I get his confidence up? He also doesn't bark which I need him to do just not excessively. I am wondering if he even knows how.