Year four is when the cracks start to show.
Oh yes. When they are nine.
There is something about nine year olds, that is very different to seven and eight year olds. They start to understand that someone is different, and they start to deliberately wind up the children who are easily wound up, and they start to exclude the children who don't fit their mental mole of normal.
Ds1 has asd, and ADHD. Year three was a dream. Year four .... Oh god, I was nearly ready to withdraw him.
Luckily for him, he has a diagnosis, and a statement. I could go into the school and say "you are LEGALLY OBLIGED to prevent him from floundering socially, you have been told by his paediatrician that he is vulnerable, act or face the law."
Currently, sgb, you don't have that legal recourse.
If your school and his peers are as good as you say they are, he may have an easy ride for primary, but this will cause an even bigger shock in secondary. Teenagers are quite capable of being utterly fucking vile to an eleven year old boy who flaps, and if you have no legal recourse to force the school to act, to provide a safe space at break times, to provide additional pastoral support, then you could have school refusal on your hands.
It's your choice, you're his mother and it will be your choice until he's eighteen, but as a parent of a child in year five, let me tell you I am extremely glad I got a diagnosis in year two.
I cannot believe how much easier my second son has found school, despite being a clinger, despite never wanting to go because he doesn't want to be away from me, he GETS social things, he gets his peers, he gets that if someone asks for his money he shouldn't give it to them because he won't get it back, he gets that when the teacher says "children, come and sit down" she means him too.
It's little silly things like that that make his life easier than ds1's, and it's little things like that which mean the sudden loss of a naturally inclusive teacher and kind peers can be devastating to a child who DOESN'T 'get' it.