What scores did your son get when he was diagnosed with dyslexia?
I have a DD with severe dyslexia - when I got her to try 11+ papers, the VR was a complete nightmare... all the questions where you need to work out the missing letters... DD probably couldn't read or spell it with all the letters there! And yet she scored almost very highly for NVR and VR at both dyslexia assessments she had.
I did ask the last Ed Psych how come she had a such high VR score and yet the 11+ verbal reasoning score was so awful, and she explained they had a totally different way of testing.
So it could be purely down to the test he sat and nothing to do with any innate potential or abilities.
To be honest, whatever the case with your son's CAT score, he's definitely in the wrong school so I would move him as soon as you can.
Look for schools that are used to working with children with dyslexia and can think outside the box a bit. DD is now at a very large secondary with a great SEN department. She's working almost entirely on a laptop now, everyone is ignoring her spelling and focusing on her actual content - and she's happy, engaged and getting really good grades in all subjects.
We were turned down for a place by one private school who said that they didn't think DD would thrive in an academic environment (and this was a school that doesn't claim to be more than minimally selective), so I know exactly what a nasty surprise it can be hearing something like this. From what I have heard since, I suspect it was more the case that the school had neither the will nor the resources to bothered catering for a child with any kind of SEN if they didn't have to.