As we mentioned earlier, there's really no set age when your child should stop believing in the Easter bunny. Between his own intellectual development and the presence of siblings, relatives and friends who might accidentally (or not-so-accidentally) spill the beans, he'll likely figure it out on his own by the time he's about 8 or 10 years old.
Keep in mind, too, that your child might know the Easter bunny is a myth and keep mum about it. Sometimes, that's because he's afraid that if he says there's no Easter bunny, the chocolate eggs and marshmallow peeps will disappear. Or he sees how much fun you're having pretending that a giant rabbit hid the Easter eggs, and he doesn't want to disappoint you by admitting he knows the truth. Of course, he might simply enjoy the entire tradition and hoopla surrounding it and want to keep it going.