How on EARTH did you manage a deferral? More or less completely impossible back in 2004, when my DS1, 4 years and 3 months had to start Reception.
It is a gift from the gods. If you know he's not ready; defer defer defer.
Briefly; my DS1 went into a 2 class entry infants; with it being divided into YR (him). and a YR/1 class.
He then moved into Y1, which was pure Y1 - (21 DC, split 17/4, boys/girls); and a Y1/2 class.
Then he did Y2 which was one class of Y2/1, and a Y2 class, very integrated across the board (being end of KS1).
My overriding memory was parents' evening in my DS1's Y1. Not long qualified but older, mature teacher. Her opening gambit, post pleasant introduction was 'How do you think your son is doing?'...
I said I wish he'd had the opportunity to repeat Reception. She said 'Yes, him and the vast majority of the boys in this (Y1) class', they'd all benefit.
An aside, in DS1's class was a boy, S. I became great friends with his mum. S was born 5/9. Eldest in the year.
At the end of KS1, back then, the DC were formally tested against NC levels.
The 'accepted' standard was something like 2.5. S got a 3. And was 'appropriately' lauded for his achievement. My DS, several months younger, got a 2.5
They went into Juniors. Where, interestingly they carried on using the NC KS marks (which, once you understand them, are quite useful). At Xmas, my DS got 3s in all 3 subjects.
He was exactly the same age as S was, when he got applauded back in July for his identical achievements; but no one gave a toss.
Sorry to bang on, but unless it will actively hinder your child to defer- do it.