Berrie is ds currently at nursey at the school? It obviously works totally differently up here. Dd was in nursey at the school at 3and a half and started full-time Primary 1 at 4 and a half. She was, and still is the only one in her year.
She started because she was ready, whereas I have friends who have children months older than her, who kept them back a year because they were not ready for 'proper' school.
Dd was only one in her year and the youngest at the school but being with the older children actually brought her on and she has always worked above her actual year if you see what I mean.
I would speak to his existing teacher and see what their opinion is as to how he would cope.
You know ds best though at the end of the day.