If your child does come up and ask you whether the Easter bunny is real, no matter how old he is, it's best to be honest. He's maturing and piecing together the information around him, and you should encourage this critical thinking -- as well as the fact that he isn't afraid to ask questions. He'll probably also be pleased he figured it out himself. Plus, denying it would require you to construct a more complex Easter bunny story.
Another tactic is to toss the question back at your child by asking him, "Well, what do you think?" If your child persists in getting an answer from you, however, honesty is still best -- just make sure your answer treasures the way he feels about the subject. For example, you might say, "The Easter Bunny is a wonderful, imaginary rabbit who brings happiness and joy to children and their families at Eastertime."
Sometimes, it is, in fact, best to broach the subject yourself. If your child is nearing age 10 and has a lot of friends or classmates who are beginning to tease him about his beliefs, it's probably best to pull him aside and gently tell the truth.
No matter how things unfold, rest assured that most kids have just as much fun at Easter even when they know the Easter bunny is simply a beloved tradition.