Do they have to be alive? Comb his hair forward and put a pair of glasses on him and he could be John Lennon. If he has a guitar he could take it as a prop and maybe learn some basic tab for a Beatles Song.

Claude Monet. You could get/make a fake brown beard, dab paint stains on an old shirt, get some cheap brushes to carry and print out a painting from google images. This would work for many other artists.

Alexander Fleming: A loaf of bread, test tube (or one of the plastic test-tubs like things they put on cut roses) or beaker/jar, and a lab coat or large white shirt.

Good luck!

A couple more ideas since I first answered:

Hugh Laurie (who plays house on TV), is actually British. He used to be on Blackadder.

Robbie Burns: just get a volume of poetry, some plaid, and help him fake an accent.

T.E. Lawrence (Lawrence of Arabia) could be a fun costume for a boy his age.

Julius Caesar. Gel his hair forward a bit and make him a toga with a purple band on it. Or you could get him a centurion’s helmet and (if your school will allow it) a cardboard sword.

Look like him, though. Don’t act like him.