"A collection of insults 42" joke

Driving with two wheels in the sand.

Dropped his second stage too soon.

Dumb as asphalt / dirt / a mud fence / a stump / a sack of hammers.

Dumber than a chicken / box of hair/rocks.

During evolution his ancestors were in the control group.

Ears are redirected to /dev/null.

Easier to count the bricks left than the bricks missing.

Echoes between the ears.

Eight pawns short of a gambit.

Elevator doesn't go all the way to the top floor / penthouse.

Elevator goes all the way to the top but the door doesn't open.

Elevator is on the ground floor and he's pushing the Down button.

End of season sale at the cerebral department. -- Gareth Blackstock

Enjoys listening to telemarketers.

Enough sawdust between the ears to bed an elephant.