M....Liz M.'s summary of the NAEYC article is not accurate. Please read it for yourself.
We are in a similar situation. My daughter will be 5 in July (close to the Sept. 1st cut-off). We really love the preschool that she is in. She has no developmental delays, but we feel that she needs an extra year before starting K. So we aren't enrolling her until next year...She will have 2 years of preschool under her belt before entering K. We do not have any peers in our neighborhood that are the same age as my daughter. Her friends from preschool will also be entering K at the same time my daughter is...So that helped sway our decision too.
I would only send your son if all of his peers in your neighborhood were also entering K this fall. He should stick with his peer group as far as when to start K. Also, if you aren't set with a terrific preschool as an alternative to K this year, then send him to K.
We love the benefits that our daughter will be receiving this next year in preschool. So we feel that it will benefit her in the long run to delay her entry.
Ultimately it is up to you. Don't worry about what other people think. Do what you know in your heart feels right. You are his mom, and you know what's best for your child. It is easy to get swayed by other people's thoughts on what is correct/standard. Just wade through all of that, and decide based on your child alone...not on what other people are doing.