noobydoo To clarify, your son is currently 3y1m? (and I'm not sure what extension days are, sorry)
My DS is beginning school this September, and last September when we were deciding where he would go, I was despairing at how young and dependent he was, completely still a baby really, especially if I compared him to his older sibling at the same age. A year on, developmentally he is a completely different child, one that is ready to begin school, though I'm sure there'll be some wobbles initially.
I do know children that have remained in Nursery class when their chronological peers have moved to Reception, however without exception they have all had some form of special need- global developmental delay, or FAS, ASD etc, and already had Statements in Nursery.
The difficulty comes when you deem them ready to begin Reception- schools in our area just do not have places, there has been a huge baby boom here, and many schools already have bulge classes (we're talking almost 20% increase in birthrate btw!)- it's hard enough to get a place at the usual admissions time. Children with statements should have an appropriate school place written into their Statement, so find it less difficult to gain entrance to a school, though those children I know that have remained in Nursery, all bar one stayed a whole year behind, rather than moving on to Reception part way through an academic year.
I do worry when children don't start with their chronological peers in September that they will miss out on friendship groups forming- a huge part of Reception is the socialisation aspect, and it can be difficult for latecomers to become part of an already-established coterie.
If your son is bright, I doubt he will struggle too much (my own DB was just 4 and 10 days when he started Reception, he was reading by Christmas). I would suggest doing activities with him that promote independence- teaching him to get dressed/undressed by himself, including outdoor clothing, toileting independently (including wiping), using cutlery, recognising his own name, etc. Make sure his clothes have fastenings he can manage (very easy to get velcro shoes these days, can't remember last time I saw lace-ups on Reception children, but make sure he can do up his coat, shirt if he has to have one etc).
And don't worry... even September-born children can find it difficult to sit quietly on the carpet and do good listening! 