I know very little about kids, but I have a few techniques to make writing easier. These techniques got me through my PhD, and I think those techniques could be adapted to kids. If you'll bear with me, I'll use examples from my PhD first, and then offer some ideas on how to adapt it to a young person's essay. You may find these tips helpful yourself.
Have a specific person in mind when you're writing. I found it easier to write my thesis when I imagined myself talking directly to my advisor (or sometimes my cat!). You could ask your son to tell you, or his favourite toy, or a pet, what he knows or thinks about the subject.
One challenge with writing is that are trying to think what we want to write, and how to phrase it, at the same time. So focus on the what first by jotting down ideas as bullet points rather than complete sentences. For your son's essay, that might look something like.
- the teacher
- the bus ride
... and so on. The next step is to write a sentence or two about each of those things.
Another difficulty is that we use more formal language when writing than we use when talking. When I found a section of my thesis difficult to write, I would explain it to someone, or write it as an (informal) email to my advisor. Again, ask your son to simply talk to you, or a toy or pet, about the topic. After he says something, you can say "That's good, why don't you jot that down?" If necessary, he can rewrite the sentences later. I tell my colleagues "It's easier to make clear writing more formal, than it is to make formal writing clear."
One big advantage of computers is that when writing, we don't have to start at the beginning, and write in order all the way to the end. Got an idea? Jot it down; you can figure out where it goes later. If your son isn't using a computer to write this, he can put ideas on index cards, and then shuffle them as needed, until he's ready to write the essay for real.