What kind of things can he write?
Could he write down the story of the three little pigs, for example?
Could he write a shopping list of snacks to buy for a family movie night?
Could he write instructions for an alien to tell them how to use an Earth toilet?
If he can, then it's not extended writing per se. It's not processing speed. It's either generating ideas, structuring his ideas, or handling the risk of "maybe not being quite right" and how that makes him feel.
Basically, a lot of autistic kids find precision comfortable and imprecision scary. It's related to flexible thinking. This can morph into correct is good, incorrect is bad. And you are at risk of "incorrect" when you are asked to be creative or give an opinion.
First off, I would work a lot on growth mindset type ideas. The idea that effort is as important as results. Praise the process, rather than just the result. "I love how you have thought so carefully about how to mix the green paint" rather than "what a lovely tree, dear".
Model making mistakes and surviving! Model your thinking process when you are unsure. 'i don't know what to make for tea tonight. I know I've got some mince that needs eating. What could we do? Shall we look at what else we have and find recipes online?"
Model feeling anxious and coping - after the event. "I was a bit worried about the dentist, I thought I might need a filing and I didn't sleep well because I felt anxious. But when I went in, the dentist was really kind and he said I don't need any fillings this time, so I soon felt better".
I taught my lad the seven basic plots of stories and how every story in the world is one or more of those plots. We used to invent a story on the way home from school each day. I would model how a story is basically an ordinary day where something unusual happens and we would have a go based on what we saw on our drive home. "One day a young man was riding his bike down the road, when...." a giant hole opened up in front of him, or his wheel fell off and rolled away, or a burglar came running out of the house he was cycling past....it was quite fun. Plus lots of stories at home, made up ones as well as books.
Talk to him about brains and how the curriculum changes and how everyone's brains find some things easier and some trickier, and that's normal. His brain is great and remembering and learning information, but finds it trickier when he had to come up with a new idea. That's ok, it's not his fault, he's just as clever as he used to be. And he can learn how to do it. Practice makes progress :)