Your son is 6. Don't go overboard with the emphasis on structure, etc.
Every story has a beginning, a middle, and an end.
Those are the only 3 elements he needs to worry about.
The other thing about creative writing is that the more describing words, (adjectives) the better.
Help him with the actual simplicity of it all.
If the subject is a tree...
The tall tree stood in the sun with lots of orange leaves. The wind came and the leaves blew away. A boy raked all the leaves into a soft pile and ran and jumped into them. He had fun playing in the leaves.
The end.
He's only 6, they're surely not asking for a Tolstoy novel.
In one minute, he can close his eyes and imagine.
What does the topic look like? What does it do? How does it make him feel?
My son used to freeze up too when it came to writing, but the thing about creative writing is that, especially at this age, they just want imagination.
Keep it super simple at first.
Beginning, middle, end. Use describing words.
I'm sure the teacher is only asking for a few sentences.
No pressure.
Best wishes!