Hi...at our local primary school, the nursery students also wear the school uniform. Of course at this age they just consist of polo shirts, jumpers (pull over or button up) and black trousers for boys or skirts for girls, black shoes. The girls can also buy checked dresses in the school colors (ours is bright blue here.)
The cut off date here is in Sept...the first part of the month I believe, which would mean that your son would be going into Reception if he is turning five in Sept. or year 1 if he is turning six this Sept. Either way he will be one of the oldest, which is good! My twins turned five in July and just finished Reception. (Sort of like US kindergarten.) They did fine, but I would worry about them being the youngest if we were planning on staying in the UK forever.
Hope this helps?
Vicki