If the school refuse to have him assessed, I would strongy recommend you go down to the private route and get him assessed yourself.
Dylsexia Action are very good and very switched on to dyslexia (obviously!) They have their own private EPs who will be able to assess very quickly. It'll just be a question of arranging a suitable appointment time. They have branches all over the place, so you should find one near you - but they are about £400+. Here's their website dyslexiaaction.org.uk/
EP should be able to test for memory issues. It could be possible that your DS has auditory processing disorder. These all tend to go hand-in-hand with dyslexia - as does dyspraxia. Do you think your DS is dyspraxic too? EP won't be able to diagnosis auditory processing or dyspraxia, but might be able to give you pointers. (I am not diagnosing via the internet btw - but they do very often go hand-in-hand)
As bjkmummy says, this will give you the diagnosis but then you'll have to fight for the support.
I really don't know what the current thinking is re: hereditary. I am dyslexic (diagnosed at 11), my DD1 is too (diagnosed at 17) - as is my DS1 (diagnosed at 6). Looking back in hindsight, I'm sure other members of my family were/are too.
Getting angry and frustrated is another problem for dyslexic children to have to cope with. In my DS's case, I was told to get him the right support asap before he became disruptive because it was become evident that because he couldn't cope, he was starting to become obstructive (but strangely, not disruptive). The obstructive side of his nature was getting stronger because it was becoming one of his main coping mechanisms.
When DC was in an indie mainstream school, he ended up becoming very anxious and I had to remove him from the school to home ed because his dyslexia was so severe.
With DD1 - who as a child was very compliant and polite - she sailed under radar and was totally failed by her secondary school (primary was excellent). She sat there quietly not learning and doing nothing. As adult, she's done really really well and is very successful in her chosen career, but I honestly don't know why I even bothered send her to school because of the amount she didn't learn!
I would strongly recommend getting a diagnosis as soon as you can. Now your DS is 9, the clock is ticking before he starts secondary.
My DS dyslexia is so severe that I had to fight for him tooth and nail at Tribunal and he is now in a tiny indie ss for dyslexics. Now he has the right support, he is thriving.
Good luck!