Can’t take both hands and get 1 finger up his ass.
His train of thought left him at the station.
A few pickles shy of a barrel. (Pat McManus, the only one I can remember from one of his essays where two of this friends were trying to one-up each other on this sentiment.)
Dumber than a stump.
Got his head so far up his ass he needs a glass belly button to see out.
Can’t find his ass with both hands.
(bolding mine)
And a road map! 
See post 52. 
But you’ve all left out “… and a GPS”. 
Paralyzed from the neck up.
Failed the I.Q. test.
Not the full quid (or the equivalent “60 cents on the dollar”).
So dumb he tried to wake a sleeping bag.
Slower than a herd of turtles.
When God handed out brains, they were at the end of the line.
Couldn’t get a clue if they were standing in a field of Clues in heat, wearing Clue musk.