I was going to tell you that in my class I have two children who are technically 'too old' for it (one by three months, one by a week), and one who is techinically 'too young' (by a week). I think cut-off dates should be guidelines and not treated as a law. Children don't grow up on a set timetable. And Torin Finser, in his book 'School as a journey', mentions having a five-and-a-half year old in his class. It's in the first chapter I think and you can probably read it on Google books.
That said, and at the risk of sounding like a Waldorf teacher, your school's cut-off date is very late! I thought I was very progressive, being willing to consider any child who had turned six by September 1st... but December? I just can't imagine teaching a class where a good deal of children (1/3th of them) are just five... I do think five year olds belong in Kindergarten. I am not going to go on about the whys and wherefores of this, since it is not what you are asking for, and I don't want to be annoying, but there is a thread here
where I talked about first grade readiness and why later might be better that you could read if you felt so inclined, and other people chimed in. It might be worth reading. (It's not too long.)
Finally, when a child's birthday is close to a cut-off date, or there are, for whatever reason, doubts about their readiness, it is good to wonder: is this a child who will be better off being one of the youngest children in a class, or one who would benefit from being one of the oldest? Try to imagine your son in third grade or in sixth grade -- what will that be like?
There is something quite magical about that last year of kindergarten, I sometimes think it is the year when children 'round up' their development, 'catching up' with whatever aspects of it might have been lagging behind before. Of course it doesn't happen every time, and often the six year olds get bored in kindergarten for the last term, but I think it is mostly our fears and concerns that turn this into a real issue.
I'm probably not making much sense, I'm quite tired... but I wanted to reply to this today.