@VarioPerfect, I'm by no means an expert: this is just what I've learned from my own DD's diagnosis. She slipped under the radar until year 5, including passing the screening test - although her spelling was a lot more erratic than your DS!!
I was blind-sided when her teacher raised it in year 5 - and I'm incredibly grateful to her teacher for realising. I had actually questioned it all the way back in year 1 - but back then that teacher didn't think there was a problem. Her reasoning was that DD didn't seem to struggle with organisation (which many dyslexics do) and that the reading would come. I trusted the teacher as the expert, and sure enough the reading did come, and I didn't think of it again.
By year 5, DD started getting very stressed as the strain of compensating increased. Funnily, organisation was indeed a big struggle for her, but no one realised because she managed through sheer determination (she's quite impressive!). But by that point the difference between her spoken responses and written work had become more obvious and the teacher picked it up.
The dyslexia assessment showed the phonics difficulty very starkly. They do things like compare reading speed for real words vs made-up words, and also read things backwards, and compare how they read and remember numbers vs words. It's reassuring that you said your DS passed his phonics screening with flying colours. My DD also passed the screening, but her phonics difficulty was always there, and it was far more obvious by year 5. Once it was pointed out, a lot of things clicked into place!
But as I say, her writing showed her difficulty far more than your DS does! But he might just be even better at compensating.
That's a really great piece of writing from your DS aged 6! I thought you said his spelling was terrible?! I mainly see completely expected errors like there/their, know/no, where/were and tricky words like fortunate. The only one which seems suspicious is 'after the fire wears off, people where lost homes they camped in fields' instead of 'people who lost homes'.
Those are words which sound completely different - and they are exactly the kind of small non-visual words dyslexics like my DD find tricky. They read by memorising whole words (yes, all of them) and associating each word with an image rather than build the word from sounds and 'hear' it (which allows everyone else to use very specialised, fast language-processing brain circuits when reading).
You could just keep an eye on it - but as I say, the earlier you discover dyslexia the better. Year 5 was quite late for DD, and she did suffer from it. Others only find it in secondary, which causes a lot of heart-ache. I do feel sad that I didn't realise, and guilty that I got impatient and thought she wasn't trying when she was working incredibly hard but it was something she was genuinely, simply not able to do. I still sometimes forget that sonething will be hard for her (!) but I quickly remember when it happens and the diagnosis gives us useful insight into different approaches to try.
DD is in secondary now, at a selective private school. I do think an academic school has been right for her. Despite being very selective, there are quite a few children with dyslexia and other SN and the school are very proactive and fantastic at supporting them and adapting things. I don't know whether that would also be the case in a super-selective grammar.