No, perhaps he wouldn't have got much help at school. There always was and still is basically no provision for academically able, high IQ autistic kids. It may have helped with his self-understanding though, to know why some things are much harder rather than feel like an alien much of the time as many of us did!
The past cannot be changed but processing it together, having a heart to heart, may help you both? With so much more understanding of ND in the last few years (at least on the surface!) it may be hard for him to imagine what it was like then, that autism wasn't on your or teachers' radar at all etc, so may help to explain? It's hard as a young adult to realise how much things can change in 20 years.
My children are still much young and find it astonishing that there used to be 4 TV channels, that mobile phones weren't commonplace until my late teens, that there was no video calling. When we drove past an old phone box repurposed into a book exchange I had to explain what a phone box was for. 🤣 Things have changed so much, it may be difficult for him to understand that it wasn't a choice you made not to get him diagnosed but rather that you had no idea about it at the time, and that even if you had tried to probably the school wouldn't have helped him anyway.
I guess at the age he is, he is trying to get to grips with who he is as a new adult so it's a natural time to look back and try to understand your childhood (and yourself) and ask questions.
Has he looked at right to choose options, if he now wants a diagnosis? That may help him to access it more quickly.