I'm a tenure-track prof at a crappy football school, making less than your brother makes, but still plenty for comfort. I would never give up the flexibility I have for a different career, unless I started a business that could make millions. No salary job could entice me to give up my current life.

If he's tenure track and bored, he needs to get into different research. Maybe he needs a hobby, or to serve others more. If he doesn't think that his current job is fulfilling enough and he needs to make more of an impact on society, he has many other ways of doing so that won't cost 8 years at $200k in salary, plus med school tuition.