There are a handful of big questions that I've seen on this site, and "is my child ready for kindergarten" seems to be one of them. My experience is that all of the skills you are worried about with your son kind of just come naturally for the child when s/he is ready to learn it. I've seen in my own household that my son was hesitant to write letters and then one day it clicked. Shortly thereafter he was reading short books.
We play games with the site words - I've made flashcards and thrown in some important extra words like booger and sushi. :) We take turns making silly sentences out of the words like, "I like to eat monsters." or "I see boogers."
I leave notes for my son in his lunchbox, on his bed, ... where ever... that he wants to read. Sometimes these are from "our neighborhood pirate", sometimes from "ghosts", sometimes from me.
His class is divided into different snack tables each day, and I asked him to write out who he'd want to sit at his favorite table.
I bought him a digital watch and we practice telling time. So, he learns to identify larger numbers like 50. We also practice saying addresses when we are out on walks.
Several times we've had a treasure hunt where I've hidden clues and activities. He gets the next clue after he... (writes his name 5 times or hops on one foot or writes a difficult letter) Sometimes we draw maps. ... He leaves us clues...
The key is to make it fun, and don't stress out about it. You might also pick up a copy of the book Outliers. If your son is young for his class, you'll really be fighting an uphill battle if he isn't ready, yet. It might actually be best to cut your losses, work on what you can, and hold him back a year so that he can excel NEXT year in kindergarten. Kids do really learn on their own schedule, and although I work to make the learning exciting, it really succeeds in our house because my son is ready for it.
In talking to other educators about Kindergarten readiness, the kids who are most successful are able to sit and concentrate for extended periods of time. This is a skill I've seen my really energetic son acquire very slowly over time. And sure enough, the assignments he is struggling with are the "sit and concentrate" or "boring" worksheet kind. He'll write 9 perfect words and the tenth he tells me "starts dancing!" Or he'll very nicely color in his pictures and then when his concentration fails - break out a black crayon and go wild.
Best of luck. It sounds like you are really doing a good job with your son. Hear your teacher out - she is an expert - and may have some good advice.
So, I'm saying, if your son is a little bit young, keep working with him, but you might consider just repeating the year. When he is ready to learn this stuff it will come very easily for him.