Any other bright ideas? I also suggested Patrick Bateman...
Send him in a dress as The Boy in The Dress
Send him in a shirt and pullover with his hair parted on one side as one of the Famous five of secret seven or any other character from Enid Blighton or Roald Dahl
Send him in normal clothes as most modern boy characters, with some accessory to denote which one (e.g. a dragon or a windlass or whatever is in the books that the current crop of kids are reading)
Get a sheet and send him as a Roman or a poor person from the Horrible Histories
Dress him in grey and send him as the sword from The Silver Sword
Dress him in a black cloak and make a mortar board and send him as a teacher from any old school book