Thanks for all the responses! Sorry, got distracted, and forgot to check if anyone had responded.
Carrots, that is exactly one of the issues I was thinking about: when ds is a teenager, as the youngest, he'll be less mature when it comes to issues outside of school - relationships, smoking, drinking etc. Hard to think of now as he's a lovely 4 year old, but it will happen!
Also, as Carrots and Butterflies confirm, no-one ever seems to regret deferring, but some people do have doubts about NOT deferring.
Raffiiscool - nursery think he probably will be ready for school. And in a way so do I. I'm more worried in a way about what impact this decision will have on his teenage years though.
Mustdash - you're right, maturity is the key. But sometimes that's hard to work out. Ds is much less mature than my dd at a similar age.
Almostfifty - in England, research has shown that summer born children are less likely to do as well academically and less likely to go to university than older children. But in England I understand you can't defer entry in the way you can in Scotland, so while lots of August born children's parents don't like it, they can't do much about it. Obviously we do have a choice here in Scotland, and given the above research (none similar has been carried out in Scotland), then that's one reason to consider deferral.