B.,
While it is the trend to send kids later now, it is a very bad trend for many reasons. The reasoning I hear that is most inaccurate is "if your child has any learning issues they will be better able to handle it if they are older" and nothing could be further from reality.
I guess what strikes me is the very small statement you made that he is antisoical and very anxious. I don't mean to be alarmist, but frequently, kids with this issue also have others (even very smart kids do, which is confusing, but common.) The younger he is, the more advantageous it is for him if he needs a targeted, structured, learning method. Children are judged by grade, not age, and if he needs intervention, he will still need interveniton if he is older, so the sooner he qualifies, should he need it, the better off he is.
Espeically for reading issues, there ia a critical time period, usually between ages 8 and 9 where learning to decode is much easier. Beyond that point, it is far more difficult and time consuming and children begin to learn by reading and writing instead of learning to read and write. By far, if you child is academically capable and is allowed to go early, with the snipit of info you provided, I would highly suggest that you persue testing and find out if your son can start Kindergarten next year (20011.)
As an educational advocate, I see this more and more frequently. The trend to hold kids back until they are 6 to start kindergarten actually backfires for a great many children.
Incidentally, I share his birthday, and I went to Kindergarten at age 4, started college at age 17, and did just fine.
M.