Embarrass him in front of his friends. If your brother has some friends, this is a great time to mess with him and embarrass him in front of others. Bring out some childhood photos or talk about an embarrassing thing he did the day before when his friends come over, he's going to get really upset.
Nuclear War Level: Throw a glass of water on his bed to make it look like he peed the night before. Wait for his friends to come and let them find him.
Be very careful when doing this. If you're a good older brother, you're supposed to have more important things to do than mess with your younger brother in front of his friends and you might look like a loser doing that.