Good on you OP for trying to support your DS, but surely this is what the teacher should be teaching? It is certainly what they taught my DC in Y7 - what is the teacher doing about it if they are concerned about the essay writing skills despite being good at English?
Washed, Does your DS do a plan of his essay before he writes it up? If not, a brief sketch of what he wants to say in the intro and include in follow on paragraphs will help him to stay focussed on the essay question and save the unpicking and rejigging at the end. I've encouraged my DD (Yr7) to do this and so far so good!