Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. I see his situation very difficult because prostatectomy is very risky for erectile dysfunction since it can permanently damage nerves. Also depression itself leads to low sexual desire and Prozac itself has side effects of erectile dysfunction. I recommend to try to solve depression with psychotherapy and to stop taking drugs. Hope I have answered the question.Let me know if I can assist you further.