While this costume won't win the wearer any points for being fashion-savvy, it's supersimple to pull off and sure to rake in the laughs.
Once your child has dressed in the clothing and shoes, have him button the shirt all the way up and tuck it in. Then he should hitch up the pants as high as possible, allowing the end of the belt to hang loose.
Pop out the lenses of the sunglasses and wrap masking tape around the bridge before your child puts them on.

For finishing touches, insert the pocket protector and load it with pens and pencils or even a calculator. Then tape a trailing piece of toilet tissue to the bottom of one shoe, and Mr. Floodwater's set to head out on the town.