I'm a secondary English teacher.
For creative writing, I'm a great believer in just getting stuck in initially - but then painstakingly reviewing & re-drafting.
The planning comes in somewhat more if you're writing an essay. & at that point, an able student may do rather well from a 5 minute mindmap of ideas to cover. A less able (or confident) kid might want a grid of points, with the supporting evidence annotated too, before they start writing.
The trick with the more able at KS3 is to move them to level 6-7, where the writing is structured to 'lead' the reader through a series of arguments to a convincing conclusion, refuting dissenting views & making clear links from one point to the next.
You can do that by planning your essay before you start, or by leaping in, waffling, & then editing ruthlessly: the first approach is quicker & more efficient!
So I'd agree with your ds's teacher that he needs to start to see the value of planning.
That said, year 7 sets are often a bit weird - ours are streamed, so we get Maths-y kids in top set who find English a struggle & vice versa. Also, they're usually based on KS2 SATs which aren't necessarily very accurate reflections of ability. My experience is that bright kids blossom spectacularly in year 8, once we've got them in 'true' sets by subject. So it may be that your ds's class are going at a fairly pedestrian pace, & he'll be rather more challenged come September.