I understand everything you are saying about your son being afraid of his father getting mad at him. I went through the same with my girls. Whenever something was bothering them they complained to me. If their dad made them mad, I had to listen to it. Not that I minded. It took them about 2 years to finally stand up to their dad. They got to the point that they didn't want to walk on eggshells anymore. Now, he knows they mean business.
Just be there when your son does finally open up. He will one day. He just has to do all of this at his own pace.
