Is there any type of theme at all? You could do his favourite book character, or movie/tv character. A play on a nickname you have for him (ie: if you call him Bug, for example, he could be a bug). Or just look for something on sale that fits!
Nathan will be Bob the Builder, because he already has the overalls, plaid shirt, and shoes from his everyday clothes. We just bought a toddler tool belt with tools and a hat. Presto!