Originally put this on secondary education but may be more apt here.....
Firstly, my 12 year old does have a diagnosis of dyslexia as well as some mild sensory processing issues (mostly auditory and visual). He also has "aspects of ASD" but not enough to have ASD. Therefore, he has always struggled at school and received extra support. However, he still managed to achieve level 4 SATS which I believe is due to attending an excellent primary school and the fact he always gives 100% effort.
However, despite struggling academically, I always assumed he was quite bright in some other areas. For example, he has an amazing eye for detail, is very practical and creative and an amazing sense of direction. He wants to go into construction or mechanics which I assume he'd be ideally suited.
I've just discovered that he has performed very poorly across the board in Y7 CATS tests, not just in maths and English as expected but in areas such as spacial awareness which shocked me. He last did a non verbal reasoning test age 7 when assessed for dyslexia and was a good average then, so a relative strength.
The good side of this is that he is getting lots of extra help which I had assumed I would have to fight for given that he came to them as "national average". I just feel deflated rather worried for his future. Do these tests really mean he's a bit dim (I actually don't think he is!) or something to be taken with a large pinch of salt?
The teacher actually said he is performing a lot better in class than expected which she believes is down to his amazing effort and determination.
It might just be that he isn't very good at those types of tests. However, there is a correlation between performance in Y7 CAT tests and performance at GCSE.
We had the opposite problem; DS1 (now Y11) scored very high in his Y7 CAT tests and all his GCSE targets were set at A*. He is able, but not as high an achiever as the figures suggest, so his reports show that he is failing to meet the targets, which is very demoralising.
If your DS was last formally assessed at Age7 and he was diagnosed with Dyslexia at this age, it would almost certainly be worth a re-evaluation to see where development has progressed and what his current profile of strengths is.
Do bear in mind that many tests contain barriers to success, and if your DS struggles to write this is certainly one of them! Anxiety is also a barrier. This is another area where a re-evaluation could offer support, as it would create an updated profile of his experience which you and the school can both use for better outcomes.