My brother miss his dead-beat father, how can I help him?

I have a nine-year-old brother who misses his dead beat father terribly. He often cries, explaining that he wants his father back at home with us. I grew up knowing who is father is and what a terrible person he is. He had drugs in our home, he cheated on our mother numerous times, and he would not work to support our family. Not to mention the fact that he was absolutely disrespectful to us all. Many times I had to defend my mother when she would not speak up for herself. I was the other adult in the house while he ran out with women or doing drugs. My mother will not talk about the situation to my brother to explain to him that he is never coming back. I grew up not having a father and while it did hurt me, I understood that there were others who loved me. My mother is always too busy to spend time quality with my brother, so I am the one who tries to expose him to postive and new things. I hate to seem him hurting like this, and I know only a man can teach him the things he should know. What should I do?