This exemplifies a difference between British and American English. In British English, 'revise' means going over material previously studied in order to prepare yourself for an exam. The first sentence is a correct and normal sentence in British English, and the second sentence explains it. The only difference is that 'preparing' suggests preparation in a wider sense. As well as revising (going over) the material, 'preparing' might mean studying exam technique, practising sample questions, or even doing some relaxation therapy so that you're in a calm state of mind before you take your exam. In American English, 'revise' means to make corrections, so the first sentence would make little sense. I believe that the American equivalent to 'He'll be revising for exams the whole day tomorrow.' would be 'He'll be studying for exams the whole day tomorrow.'

to revise means going over things you've already learned . To prepare means to actually study something new.