No one knows who Frederick Douglass's father was. The only thing that Douglass himself knew was that his father was a white man. It is likely that his mother knew who his father was, but she was sent away from her son when he was very young and so she could not tell him.
The rumors that went around when Douglass was young (and which are credible because this would not have been uncommon at the time) was that his father was the man who owned both him and his mother. This was a man named Captain Aaron Anthony.