Most Helpful Guy
Best you could do is just be very honest, tell him you would love to go but you don't feel well and that you think that sucks. Maybe be subtile and ask if he is going any other time soon, that way he should just understand you indeed do not feel good and you indeed wished to go. If he reacts bad to that well.. he should trust you on your words..
Most Helpful Girl
U tell him u don't feel well, and specify another date to hang out