If what you don't know can't hurt you, he's practically invulnerable.

He's locked in a life and death struggle with mushrooms.

His IQ=dx/(1+dx), where x=age.

He's not the brightest crayon in the box.

S p a c e d o u t.

He's so dumb, blondes tell jokes about him.

Some drink from the fountain of knowledge-he just gargled.

He has the perfect personality to write software manuals.

He's so stupid, he thinks "Private Enterprise" means owning a personal starship.

He's so stupid, he thinks Moby Dick is a kind of venereal disease.

He's a doughnut short of being a cop.

He's a hamburger short of a Happy Meal.

He has a mind like wet tennis shoes…makes squishy noises when running.

He has a natural talent for finding subliminal messages in ice cubes.

He has a room temperature IQ.

He's a titanic intellect…in a world full of icebergs.

He's a walking argument for birth control.

He always loses a battle of wits because he's unarmed.

He's as sharp as a wet sponge, and twice as smart.

He's as worn out as a cucumber in a convent.

He has both oars in the water, but on the same side of the boat.

Calling him a pea brain would be an undeserved compliment.

He can discern facts and form predictions with the acumen of an economist.

He's so stupid, he cheats when filling out opinion polls.

He couldn't count to 21 if he were barefoot and without pants.

His cranial cavity is filled with neutronic matter. (dense)

He has diarrhea of the mouth/constipation of ideas.

He's differently clued.

During evolution, his ancestors were in the control group.

He gets a charge out of pissing on electric fences.

He'd be a perfect goalie for the dart team.

He got a life, but wasn't sure what to do with it.

Guillotining him would make only an aesthetic difference.

High Relative Humidity-he's lost in a fog.