Ask if he'd rather not dress up and just take a book in. If he wants to dress up, what about:
Wheres Wally (stripy jumper, beanie, glasses and backpack)
Ash Ketchum from Pokemon books (jeans or blue shorts, white t, blue gillet or jacket and red cap, carry a pokeball /picachu toy)
A white with back spots dog onsie and go as a puppy from 101 Dalmatians
PJ and a bandage round his head and go as one of the kids from the Midnight Gang
Dress in brown with green leaves cut out of paper/card on him, carrying 2 shoe boxes decorated to look like little houses, go as the 12 story tree house
When DC1 was in year 4 he announced the space penguins was his favourite book so I sent him in wearing a spaceman dress up costume with a penguin mask and a nerf gun.