Have you identified any possible special schools? And/or have you spoken to the Sencos at any primary schools? As others have said, there are also some primaries that have a resourced based unit for children with autism where they spend time across both settings.
I don't know what it's like where you are, but where I am, there is one special school for primary aged children. It only takes children with the most complex special needs, including life-limiting conditions. My friend's DS who is in a mainstream primary and is in Yr3 is completely non verbal and still in nappies and he didn't qualify for a place. (My friend has a tribunal coming up). My point it that the criteria for a SS place can be quite high and you will need reports from specialists to say that this is the only setting that will be able to meet DS's needs.
My DS was non verbal until almost 4. He went to a mainstream primary with full time support. For him, it worked - although that's not to say we didn't have plenty of bumps along the way.
For me, although he found it challenging, he did learn to cope with a large class setting, to access the full curriculum (he is deemed 'academically able') and to rub alongside with his neurotypical peers.
For some DS on the spectrum, if proper support is put in place, they can cope with a mainstream primary. Secondary can be a different matter!
You will also need to think about a peer group for your DS. It's obviously difficult as he's only 3 now but as part of the EHCP process, he will be seen by an EP. They will help to identify his cognitive profile.
If/when you do get an EHCP, you can name an out of borough school or an independent school (although you'll probably need to go to tribunal as the council is likely to object on costs).