Politically Correct Ways to Say Someone is Stupid...
Doesn't have all his dogs on one leash.
Forgot to pay his brain bill.
Her sewing machine's out of thread.
His antenna doesn't pick up all the channels.
His belt doesn't go through all the loops.
Missing a few buttons on his remote control.
No grain in the silo.
Skylight leaks a little.
Too much yardage between the goal posts.