I agree that it's HIS qualifying exams not yours and not the firm's. It is hence his own personal business.
Tell him that you can't do his exams but you'll take on other tasks so that he finds more time to study for his exams.
If he pushes on, tell him that you like him but hey, you wouldn't do it for your own brother. And your own brother wouldn't do it for you either.
If your manager retaliates, then your manager has ethical issues and you probably should consider a transfer. You wouldn't want to be looking over your shoulder and wondering what other unethical demand he'll come up with next.