Actually, I'm pretty sure this is about the older brother of the biblical Prodigal Son, the one who didn't run away, hence the good son. I guess, judging from lines like "they are deaf in the shadow of his brother's truancy" and "he curses his virtue like an unclean thing," that he's disappointed in not getting credit for being obedient and wishes he'd been the one who rebelled, since the prodigal son so unfairly gets all the love from their parents when he returns.
Actually, I'm pretty sure this is about the older brother of the biblical Prodigal Son, the one who didn't run away, hence the good son. I guess, judging from lines like "they are deaf in the shadow of his brother's truancy" and "he curses his virtue like an unclean thing," that he's disappointed in not getting credit for being obedient and wishes he'd been the one who rebelled, since the prodigal son so unfairly gets all the love from their parents when he returns.