Just wondering-- have you spoken to your son's preschool teachers about this? When my son was newly five, I asked his preschool teachers what their opinions were. (They were both very seasoned and had high school and college-aged children of their own.)
Some things I would consider would be the following:
Is your son able to follow the teachers and group through the day, and how are the transitions and self-care tasks going? Does he need a lot of help, or can he manage himself?
Is your son able to self-regulate his impulses and refrain from hurting other children when he is frustrated? Can he stay in the group setting during gathering/circle times at preschool?
Honestly, these were my two big concerns as a parent. I wanted to make sure Kiddo could move through the day with the group and I knew that it was important that he manage his body and emotions so that the teacher would be able to do their job. Personally, I did not sweat a lot of the literacy/math stuff, but just introduced those ideas as games. At the kindergarten age, it actually takes teachers more time to teach children to take care of themselves and to follow the group through the day than it often does to teach the actual course material.
If you google Kindergarten Readiness, you will find tons of information.
For what it's worth, kids learn what they learn when they learn it. My son knew about 12 letter going into kindergarten (I am a preschool teacher myself, and knew he wasn't interested)... five months later and he's reading simple words now. I urge you to keep any readiness prep fun, and to keep your prep very light.
As I said, I checked in with the preschool teacher, but my son turned five in April and I just figured he'd be okay. I was actually the youngest in my class--- an October baby, who started in Hawaii, where the age cutoff was in December in the 1970s. My husband is also a younger one, same circumstance in New York. We did okay being younger. It's what you make of it. We figured he would be okay--or not, and then we'd find some help for him. You just have go forward doing what you know in your gut is right for your family and your child.
ETA: someone on this thread asked the question "what have the parents been doing with their kids up until that point?" (in regard to readiness... and I am *only addressing letter/numeral identification* here, the other qualities suggested are appropriate-- but I really don't want parents to fret too much on letter/numeral identification, because this is one that keeps coming up)--I can assure you that even parents who do introduce these concepts (like myself) don't always have kids who absorb this information in *our* time. Kids have different aptitudes...kids can learn a lot of math and language concepts through exploration and play without having all of the 'symbols' internalized. Doing activities which provide relevance and meaning for the ideas presented augments the experience of using those symbols later on. THIS is precisely why I did not stress when he didn't know so much in this area-- and why the teachers who assessed him at his school suggested he was ready-- even without complete numeral and letter recognition. I felt that if he knew just part of it, he could go into kindergarten and have plenty to engage him and to learn. I just wanted to add this so that parents who have kids who aren't quite proficient yet do not think that it is a requirement for being successful at school. I prefer to spend my time with my son doing things we both enjoy like exploring our world, going on little adventures, cooking, doing puzzles, poring over the oversize books at the library on topic we are interested in, building things and the like-- not worksheets. (I'm not suggesting that's what DVMMOM did, but that's what *we* would have been relegated to.) He'll get a million of those, and for him, they do not cultivate a love of learning. The experiences, however, do.