We used a clewer diagram when I was learning this. Being the big writing/english devotee that I was/am, I still remember it, almost 30 years later....
The first paragraph of an essay starts as an inverted triangle, pointing down like an arrow. The "top" then illustrates the broad first sentence, gradually narrowing down to the "point" of the essay, which is the thesis statement.
Three squares follow to illustrate the "prongs" mentioned by tiff above. Use those "squares" to work through the idea, citing statistics, examples, etc. I believe you could state the main idea in the first sentence of the paragraph and then again at the last - reword it, of course - to tie it all together.
The final paragraph is a triangle sitting on its flat side, illustrating starting with your thesis statement at the point then broadening (is that a word?) out to a very general, but thought-provoking final sentence.
Maybe a visual like this would help him see what it needs to look like? Also, you could probably read some essays to see how those authors organize their thoughts. I wouldn't recommend starting with CS Lewis, but that's who I think of first.
Let us know how it goes!
Wih blessings, Jenni