He's as smart as bait.
If his IQ was two points higher, he'd be a rock.
He's a pepperoni short of a pizza.
I'm busy right now. Can I ignore you some other time?
The receiver is off the hook.
His tree is missing some branches.
He's about four cents short of a nickel.
Someone blew out his pilot light.
He has bubbles in his think tank.
He has a one-bit brain with a parity error.
His train of thought is still boarding at the station.
He's so dumb he thinks cheerios are donut seeds!