Class 8 son writes 90 words when the teacher wants 250, and says he has said everything

His English teacher sets a composition every weekend, about 250 words. He writes 90 or so, all correct, and says he has 'said everything'. Last week's was on his favourite festival: we go to my grandmother's, there is food, we burst crackers, we come back. That was the whole piece. I don't want him to learn to stuff sentences with 'very' and 'beautiful' to reach a number, which is what the guide books seem to teach. What does ordinary weekly practice look like for children who learnt to write at length? 'Read more' is the answer I keep getting. He does read, so I suspect the missing step is something more specific.