....and act them out.
Often when a child is telling a story and acting it, it is easier to encourage better language. For instance, they may say 'Ben walked through the door', when in fact they were creeping, so you can encourage better vocabulary.. crept, tiptoes, etc.
Also, we concentrate a great deal in question words: what, why, how, when, where, who. Start with a simple sentence,(i.e.) 'He felt sad'. Why did he feel sad? Who was he? Where was he? What did he look like? What was he doing? When was all this happening? What could he hear? etc, etc.
Answering only a few of these questions, you could end up with:
It was night-time. The owls screeched as Ben sat in the gloomy turret, waiting for the clock to strike twelve. He felt lonely and miserable.
You don't have to write anything down - you can build ideas orally. It just gets children thinking in more detail, using all senses and expanding their vocabularies.
'Think of a better word for....' is always useful, and if he's so inclined, you could keep a word book from new words that your son comes across. (I say 'as he's so inclined', because I know my son would have rolled eyes and thought I'd gone barmy for suggesting something like that when he was in Y4 (and years 5,6,7,8,9,10,11, 12 and 13 for that matter! )