Your translation is more natural English by far. So many of these discussions over translations into English are arguments about how literal or colloquial a translation should be, and they are tiresome. The problem arises, I think, because so many of the course developers are not native English speakers. The purpose of the course is not to teach English, after all.
Previously someone answered the opposite, and with зачем, the answer was what does he read that much FOR? what does he do that for, for what purpose? So Im still confused. Purpose: he has a test on it tomorrow. Cause: he just loves to read, or his mother told him, if he doesn't he is grounded.
I am pretty sure the phrase «так много» is like "so much" whereas «много» alone would be more like "much" or "a lot".
Зачем он так много читает? = Why does he read so much?
Зачем он много читает? = Why does he read a lot?
Similar sentences but different meanings. (I hope a native Russian speaker will correct if I am wrong.)