B.G.
As a former preschool teacher, we helped parents a lot with this decision. Ultimately it's up to you, but I once heard someone say, "If you could give your child one more year of childhood, wouldn't you do it?" I thought that was a unique way to look at it.
My husband was an October baby, and one of the youngest in his class. One of the things to consider is when your son goes off to college, the fact that he will still need your signiture for several things, not yet being a consenting adult himself.
My daughter is an August baby, and I know she will be the youngest in her class. At least you have the option! :)
Good luck with your decision!