My son is finishing y2 and is reluctant to express himself in writing. I think partly it stems from being left handed and his historically finding it difficult but this issue could impact a right handed child who still struggles to get images from their mind out through their pencil.
The other issue is one of confidence. My ds is self conscious in other ways.. doesn't like to be out of uniform on dress up days. He doesn't like to 'let go' of himself publicly.
I don't have specific advice for a child of your sons age but I'll tell you what I'm planning to do to help my ds.
Over the summer, I plan to write a sentence a day with him and then elaborate on it together as suggested by previous posters to incorporate alliteration, simile, personification etc.
I don't plan to make it an onerous task. Short sentences are fine etc.
Then I'm going to collect the sentences and use them to make a funny nonsensical story.
My ds likes to read to our cat. I'm going to start our sentences probably by describing the cat eg: "my cat drools" to add "when he is happy" to add details of what his drool smells like (which is a talking point in our house!) But how we love him anyway etc. When you rest your head on my cat you can hear his purr "it sounds like X" or 'his fur is always on my uniform' etc.
I hope this discussion of something he loves combined with creating a story or a love letter for the cat will help.
Finally, my son loves rap music and although we struggle to find clean versions of rap to listen to, I think I'll be able to adapt cuts of lyrics to discuss with my ds. He loves Eminem's Lose Yourself and there are a lot of examples in there that I can tweak his interest in. Eg:
"His palms are sweaty, knees weak, arms are heavy
There's vomit on his sweater already, mom's spaghetti" (from Eminem's Lose Yourself)
Rap is also an example of how there is nothing wrong with short factual sentences that build on each other to make a story that way.
I might struggle to find many appropriate rap songs so might try Katy Perry's Roar!
I hope that helps. Sorry my post is so long but it's difficult to share the ideas without elaborating.