When you're talking to each other quickly avert your eyes to behind him and look a little alarmed. When he says "What?" say "Huh?", then check your watch. When he calls you out on it deny it. Continue.
When he's sleeping or resting (if you are likely to be around) sneeze really loudly and spray him with a spray-can in the face.
Wear a cup and tell him you have fallen in love with his sister. Tell him you are engaged.
When you are at school, steal his car keys and park his car in the teacher's spot. (Kind of mean and could backfire, pick your teacher wisely.)
When you are at school, take his lunch out of his bag when he's not looking and take a bite out of it. If he catches you say "TASTY!" in your best Jim Carrey.
