at school I was always told I had no imagination or creativity. I struggled with things like writing stories. I ended up studying sciences and had a real skill for being concise and for being able to summarise but my creative English was always an issue. I got Bs for English Language and English Literature at GCSE so I did manage to work with it but it just didn't come naturally to me. For some people it doesn't and I am sure like you have said, he will have his talents in other areas.
A few ideas which may help. Perhaps see if you can borrow some of the wordless picture books from school that they use in Reception and photocopy the pictures so he doesn't know they are from younger books as that may seem insulting to him. He can then use them as a story board. Some of the Collins Big Cat books have a story board at the back of them, the school may have some that you could borrow. He can then use the pictures to tell his own story. start with the basics of what is in each picture and then add more words to it so he learns to make it descriptive etc. gradually you can reduce how many scenes are in his storyboard so he has to make up more of the ideas himself.
you could make up storyboards with anything, photos or pictures from magazines.
you could cut out or draw a few pictures of locations, a house, a beach and a park for example. then draw some people and animals that he could choose some of for his characters. then perhaps some objects to use say a chair, a bike, a magic candlestick or something like that. start with things he likes. then talk about different possible things that the characters could do.
It is really about building his confidence. He is probably a perfectionist so wants to pick the best idea to write the best story he can but then he worries that what he is choosing might be the wrong thing. That was always my worry, or that people would think my ideas were silly.
also reading lots of other stories will help him build up a bank of ideas in his mind so that if someone says they want him to write a story about a lost parrot he can think 'oh I know, the parrot flew off on an adventure and did whatever'.
hope that helps a bit. I really do think the main thing is about building confidence.
the other thing you could try and I have no idea if it would help or not but worth a go. buy him a 'secret' notebook which is just for him, and tell him it is for him to write stories in, noone will look at it, he can hide it where he wants to, noone will ever read it unless he wants them to but he can play around with ideas in it until he builds things up.