I would do fun stuff
pick work he enjoys
an old paper or just parts of a paper that he will whizz through and get a high score.
Something NFER style multiple choice if possible, talk about exam technique and focus. Lots of praise for all the hard work completed to date, do your best on the day, keep going, if it is hard then everybody will be finding it hard so just do your best. That was my speech three years ago, just rehearsing for next year.
It is all about having a good day, on the day.