There are a collection of metaphores that refer to a persons intellectual or neurotic quirks:

Everyone has heard, "He's not playing with as full deck."

You may have heard the more hip, "He's not the sharpest knife in the drawer."
or the similiar, "He's not the sharpest tool in the shed."

My all time favorite, "He's one taco shy of a combination plate."

I just heard a new one, "Chalk don't stick to his blackboard."