Hi OP - you are in exactly the situation I was in this time last year. My DS has an ASC. While he wasn't speech delayed, he was/is behind his peers emotionally and developmentally. He wasn't even potty trained this time last year and as for reading and writing - he was still scribbling and couldn't write his name. He is Sept born so almost a full year older than your DS.
We had decided to keep him in his nursery an extra term and do a very gradual, staggered entry into school. I had spoken to both his school and nursery and they were both fine with it.
Be aware that although it is your legal right to do this, some schools aren't keen. The rules around school funding have changed so if you keep your child at nursery it's they that get the government funding for your child until they leave. If a school is stretched for money, they may not be happy as they will have to find budget to make up the shortfall. A good school should support your decision though as the welfare of the child should come first.
Our decision changed though when we found out that his LSA at nursery was leaving at the end of the summer term. We decided there was no point in him staying at nursery for the extra term as all his peers and his lovely LSA was going.
Now fast forward a year and I am so glad I sent him in September.
He's so happy and settled - he's even made some friends.
Also - it's given the school that bit longer to work him out too and to put in extra support for him. He does some small group work to help him socially and academically. The jump from Reception to Y1 is pretty big (from what I've been told) so the more time they have to enjoy Reception and figure out how school works, the better - I think.
Have you met with the school SENCO yet? I would advise you to call the school on Monday to set that up ASAP. Or get your nursery SENCO to do it.
Also - have you told your son's nursery that you are considering keeping him there? If you haven't, they may have already allocated his space for September so you'll need to speak to them ASAP. My DS was at a Sure Start nursery which meant they had to keep him but I'm not sure how it works with a private setting.
I had sleepless nights trying to work out what to do for the best. It's such a hard decision to make. Good luck.