It greatly depends on his personality and why/when/how he is shy. But one of these might work:

-- Remind him that the dance is about a boy and girl liking each other and he should try to show the audience that he likes his girlfriend through his dancing

-- If he's not comfortable expressing emotion directly, he might feel more confortable thinking of the performance as acting.

-- Approach it purely physically. Give him some explicit styling moves that add some pizazz to his dancing and teach him to do them with emphasis on the technique, not the emotion (best if he is a very shy person in general and is made more nervous by the idea of expressing or exposing himself through dancing).

Good luck! There may be only so much you can do in such a short time--but the family will love the performance anyway!