Why does Les call sin 'the old Adam' when teaching Romans?
Sin, in the singular, is called 'the old Adam' throughout Romans because it refers to the Adamic sinful nature—the fountainhead of all our problems. Paul uses singular sin for that old Adam, while plural sins describe the deeds old Adam causes us to do.
Les explains that he uses 'old Adam' in place of the singular word 'sin' throughout Romans because that is what Paul is really talking about. When Paul speaks of sin singular, it is that old Adam, the fountainhead of all our problems and the Adamic sinful nature. The fruit of old Adam are the things we do—plural sins—while sin singular usually refers to old Adam and sins to what old Adam causes us to do.