Depends on where you live.
In my county and NHS GP won't touch anything to do with getting a diagnosis for a child with autism.
There are two routes - either you go down the private diagnosis route - this has the complication of whether that is officially recognised by the school / the NHS. (In most cases it would be, but not always and that can add complications but it's often quicker) or you go through the school referral system.
Basically your school has to refer you on through the NHS service - this is because they have a period of 'observation' in school.
However the next county over do not do any diagnosing / referral system at all. They purely treat kids on the basis of whether they think they are neuro-diverse or not. This has its positives and negatives - it means that kids with the biggest difficulties immediately get support they need.
But it comes down to funding. The schools have to think it's worth applying for funding and that depends on their criteria for that.
In my county it's more difficult to get funding without a diagnosis. But if you have a EHCP they still can get some. Next county over works differently (not entirely sure how it works).
So a) you need to find out how the system works where you live rather than listen to a pile of irrelevant advice on MN from people who live in areas with a completely different system b) look into getting an EHCP.
Other things to keep in mind - unless there is a significant problem (eg mutism) DS's school, usually won't refer until a minimum of age 6. And the process seems to be around 18 months - 2 years long here. DS's don't tend to suggest a child is neuro-diverse to parents - they hint at it until you take the bait and the parents takes the initiative or it gets to a point where the child is referred all over the show for significant disruption or behavioural issues affecting the other kids. In other words it's down to parents to initiate conversations with the school asking the question about this rather than waiting for the school to say something. It's arse about face and to do with the fact the local system is free for one child per class per year and then costs the school for each referral. So they want to prioritise and delay...
Either way you need to arrange a meeting with the teacher / senco and be direct!