Need less offensive way to say "he doesn't know the difference between his ass and a hole in the ground." [closed]

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.)

Solution 1:

As stated in a comment, I typed "doesn't know his" into Google to see what it came up with. It liked "head from a hole in the ground".

From Wiktionary

know one's head from a hole in the ground

Synonym of know one's ass from a hole in the ground

Solution 2:

What he doesn't know could fill a book.

This is an idiomatic way of expressing the breadth and depth of one's ignorance. It should be used to express someone's lack of knowledge in an area that they ought to know something about. If you want to be hyperbolic, replace book with library for greater effect.