I'm writing something in a tone that is just a little too formal for "he doesn't know the difference between his ass and a hole in the ground" or "he doesn't know dipshit." I need something that doesn't include words like ass or shit, but that still gets the idea across and packs some punch. For example, "doesn't know his eye from his elbow" doesn't do it for me.
(This question was already asked within a different question, but didn't get any good answers. I can't place a bounty because the main question was closed.)