My DS was finally diagnosed with autism in Yr4. As you say, it can be a long process. Where we are it was a 2-year wait and I suspect it's even longer now due to Covid.
As I said on another post, for my DS, the gap/differences between him and his classmates started to become really noticeable from Yr3/4. I was worried about secondary and knew he wouldn't be able to cope in a mainstream school - especially with no support - so I really pushed for a diagnosis and then for an EHCP.
In terms of first steps, do speak to the Senco. They can refer you on or if you get no joy there, speak to your DS's GP. It would be helpful to make a list of your concerns. They may ask you things like when your DS started to speak, walk, did he crawl, point, potty train etc? It's useful to try and jot them down - if you can remember!
Your DS will then be referred onto a community paediatrician. They are the 'gate keepers' to the ASD waitlist. They will see you and your DS and ask a host of questions and also talk to him. If they believe it's worth him being assessed, he will then be added to the wait list.
In the meantime, I would say it's worth doing as much reading as possible about tips/techniques to support children with ASD. Even if your DS doesn't get a diagnosis, many of these strategies are helpful for children who are anxious or struggle socially.
I would also start a diary outlining any triggers you notice that cause him distress or any situations in which he struggles. This will also count as valuable evidence, especially if you say he 'masks' at school.
Good luck!