What size of shoe does a 6/7 year old boy wear?

My nephews bday is coming up and one of the things I want to buy him is a pair of shoes. I don't know what size of shoe he wears and not enough time to get in touch with his parents before I'll be buying them. Does anyone know what the average size shoe for a boy who is about to turn 7 is?
Update: I wanted to add that the shoes are actually flip flops (Cars :) ) So the fit shouldn't be bad as long as I get a size that will fit. Thanks for the answers so far!!
10 answers 10