What to do when the kid i babysit has a tantrum?

I babysit this little boy thats 4 years old. Most of the time hes really nice, but sometimes he throws massive temper tantrums. Like, when he wants to eat candy before lunch. I tell him he can have candy after lunch, and BOOM! he throws a fit. I want him to stop crying, i dont want to have to give in. So what should i do?