"I don't like to write words."

Writing is not Jordan's strong suit at this point in life.  Math homework he does with gusto, spelling homework he does willingly (he no longer complains about the weekly spelling tests), but lately he has these story prompts that he is supposed to keep adding to all week long, and he really does not want to do it.  Here is last week's:

The prompt is:  "Write a story titled, 'My Journey on a Pirate Ship.' You and your friends can star in the story."  Here's what he wrote (translated):

I would find my friends.  I would drive to a faraway island and wait for a pirate ship.  It smells bad.  It was loud too.  I like to be a pirate, it's so fun.

The first two sentences were his sullen contribution - he wanted to be done after that.  I dragged him along to write the other three sentences.  The resulting tale did not pass muster with Miss Wodrich.  She returned it with that cute red-apple post-it, on which she had written cheerfully, "Jordan, this is a great start to your story.  Keep going and add to the story!  Keep it up!  :)  -- Miss Wodrich."  I read him the note and explained that he would need to write more of the Pirate Ship Adventure, in addition to writing about whether a monkey would make a good pet (which is this week's prompt).

He had a big, sad tantrum, especially when I said that he would not be able to have any computer time until he had finished his homework.  After calming down, he found it in himself to write a decent quantity of sentences on the monkey question.  That was yesterday.  Today, faced with the pirate ship, he said, "Mommy, I think I would rather write the letter."  This was an option I had offered him the day before:  I had said that if he wanted, instead of writing one story, he could choose to write a letter to Miss Wodrich, explaining what the trouble was.  Here is the result:

"Dear Miss Wodrich, 
I don't like to write words.  So I could only write one of my stories.  The reason I can't do it, well, I just can't think of a sentence!  And I cannot think of words to write.  But I do like to draw pictures.
Sincerely,
Jordan"

My thinking in letting him do this is that it's still a writing exercise; it's authentic writing, in which he is saying something he actually wants to express; he's giving his teacher a window into what's going on for him, which might help them have a more effective relationship; and he's practicing telling the truth about what he's stuck on, which I think is an important and difficult skill.  It seemed to me that he took the letter much more seriously than the original exercise.  It was also more work for him just in the sense of writing more lines of text than he would have otherwise.

It's a juicy question, though.  Should parents allow students to explain their problem with an assignment rather than doing the assignment?  Does that foster mature judgment about self, task, and teacher relationships?  Or is it more important to teach that sometimes you need to just buckle down and do things, even if they are difficult, because that's how you learn?