pls, I am sorry you feel that your child was not able to show his potential in the exams
Unfortunately, although these exams do manage to distinguish a lot of the most talented children, it is not a perfect exam and there are plenty of bright children every year who do not manage to make the mark on the day. It is very disappointing for those involved on the receiving end of those results and it does not change the potential of the child. However, it might be difficult to make a case for appeal if they were already aware of the dyslexia and you have no other extenuating circumstances.
wrt the pass mark: you have to remember that the marks we received were standardised marks not the raw marks/percentage achieved. There are threads that explain it in more detail if you are interested in searching for it on the forum, but basically it means the scores we are given relate to the distribution of scores amongst the cohort that took the exam not how many questions were answered correctly - so if taking the exam earlier meant that children were generally scoring a lower percentage we would not be able to know that from the standardised scores. If everyone put in their preferences in the same way as they would have if they had not known the scores before CAF, then we can assume that 'pass marks' would stay the same.
There are two factors that will have a more direct impact on pass marks: the main one is how parents actually fill out the CAF preferences and whether this was significantly impacted by knowing the results in advance; the other one is what we already know about more children taking the exam this year than last last year (this one is probably less significant but might just make a difference).
I wish you and your child all best at whatever school they end up at.