You're absolutely not being over sensitive.
I'm dyslexic ( & also had an 80s education) & so are both my daughters (now 21 & 19).
From my experience some teachers are very dismissive of dyslexia. They seem to think that by going over and over spellings/multiplication/punctuation enough times that something will miraculously "click" and a dyslexic child will suddenly be able to spell/know their times tables/use punctuation correctly.
This could happen with some things as every dyslexic child has differing learning issues.....but equally they could still be struggling with certain spellings/multiplication/punctuation when they leave secondary school......and that's not a failure on their part, its just how their brain works.
This teacher needs to understand more about how dyslexic children learn. They don't work at a nice steady pace. Some things they struggle with, some things they are great at....and no amount of "cramming" will suddenly make them "catch up". It just causes them stress and makes them feel stupid.
At my daughters' school they wouldn't let them write with a pen until they'd mastered cursive script. They both had to do this bloody stupid rainbow writing where they traced over the words neatly in 3 different colours. My girls would cry because they just couldn't manage it. They also had to do "catch up spellings". They felt singled out and said that everyone could see that they were stupid. Their teachers didn't seem to understand why they couldn't write "nicely" nor why they couldn't just "try harder with their spellings".
I worked there as a TA at the time and eventually I had a very "tense" meeting with in which i said that I wasn't going to make them do rainbow writing or extra spellings anymore as it was causing them more harm than good and shaking any confidence they had in their ability to learn.
If your son has a support plan & a great SENCO (my daughters didn't have either) then keep pointing this out to his teacher. Keep reiterating to her that he is not on the same level as his peers at some things and may never will be.....and that this IS NOT HIS FAULT.
Also, can I recommend a book called "Creative, Successful Dyslexic" by Margaret Rooke. We may not all grow up to be as famous as the 25 interviewees in the book, but their stories are inspiring & hopeful.