Yes, unfortunately waiting lists are very long. It's great that your school have referred you guys though.
So practically you should benefit from things such as OT sensory or motor skills assessments. Speech and language assessments. And if anything is picked up you should receive interventions and support.
Depending on your area you might have things like SENAS who can offer support with social and communication difficulties. He might be able to access support in school for groups that address these kind of things.
Your SENCO should provide more information about what's available.
Depending on the needs assessed, you might be able to apply for an EHCP or even a place on a specialist unit.
The most important thing that a diagnosis will achieve (as far as I have learned from listening to autistic adults) is the validation and self-acceptance that your son will gain from having an explanation as to why he is different and why he finds some things harder than his peers.
I think this is priceless in terms of having an identity and being able to make sense of who he is.
Again, depending on needs you might also be entitled to DLA which can help fund for any services that are lacking in your area.
For instance in our area we don't have an OT service so I pay for a private OT with the DLA money xxx