We tried keeping DS2 back temporarily and phasing his move into year 1, but it didn't work out for several reasons:
Reception class shares a common area with nursery. Whilst the play in that area was developmentally appropriate for him, the fact that he towered over the younger nursery kids and had a tendency to step on them wasn't ideal. Some of them were terrified of him.
The separation from his peers wasn't good for him. He lost touch with the kids he could happily play alongside and didn't form any bonds to replace them.
On top of that, a lot of kids from his own year group who hadn't been close to him forgot about him and a couple were quite nasty about him.
He was still developmentally and academically behind kids a year younger than him, anyhow.
The year 1 classroom was actually calmer and more structured than the foundation unit and his learning was more purposeful in there.
He's in year 3 now and has a completely individualised curriculum, as he's still on P levels. The relative maturity of the other children in his class has been an asset, as he is happy to sit alongside certain children, who take an interest in what he is doing. There are children he trusts to hand a PECS card to (he's non-verbal) and they will take him to find someone who can help and he will happily play alongside these children at playtime.