Question: 'Why must I write it like this?' 'He will be revising for (his) exams the whole day tomorrow.' is fine in British English. Another way of saying this is 'He will be preparing for (his) exams the whole day tomorrow.' From a British English perspective, the first one is better. 'Preparing for an exam' sounds a bit vague to me, as if he is sharpening his pencils or doing meditation exercises - similar to preparing for a performance. If you say 'revising for an exam' to anyone who knows British English, it is clear what he is doing - reading through his notes and going back over work that he has done during the course. One thing to bear in mind, though - American English doesn't use 'revise' with this meaning. For speakers of American English, 'revise' is only used in the sense of making revisions/changes.

Правильно будет prepare to examination (не exam). Или revise for exams. http://www.oxforddictionaries.com/definition/english/revise

He will be "reviewing" for exams "all day" tomorrow. He will prepare "for" exams all day tomorrow.