Hi OP, can you describe how your DS has regressed since being at preschool? Is there any possibility that this would continue even if he was in school?
Some children with additional needs really struggle in busy mainstream environments, even with the support of an EHCP.
It is easier to get an EHCP when your child is attending an educational setting, even if it's just nursery. They have access to specialist services such as Ed Psych and Speech/Language Therapy, even if these resources are stretched and hard to access.
Do you actually think the school is going to be the right environment for him? If he's still not verbal and in nappies, does he actually need more support ie/a special school? As @twilightermummy accurately describes above, it's an absolute FUCKER trying to get a specialist setting when your child has started in mainstream. In my (bitter) experience, it's better to aim for the special setting from the start if you think it's going to be needed.
If your DS was settled in nursery I'd defer him for a year and let nursery help you fight for the EHCP. It doesn't sound like he's ready for school yet. I very much doubt school will have the resources to support him in the way that he needs, and he sounds as if he's a very long way from where the other children will be.
I'm sorry this is such a shitshow. It is. And it won't be any consolation at all to know that you're walking a very well-trodden path. You have to fucking FIGHT and FIGHT for everything. Even things that you think will be obvious. I know the LA is short on funding but it always seems to be our SEN kids who suffer the most. The system is utterly woeful. If people haven't experienced it, it's hard to comprehend just how inadequate it really is.
For context, I have two autistic DC (and I'm autistic/ADHD myself). DS has high needs and was diagnosed just before he turned 4. My DS sounds very similar to yours at age 4, and our LA also initially refused to assess (just before he started reception class). Thankfully I managed to get them to reverse their decision by kicking up a huge fuss, and DS got an EHCP with the maximum amount of 1:1 support - which shows how ridiculous their initial refusal decision was. He later moved to special school and that was another huge bastard fight. I won't even begin to describe the fights we had for DD who wasn't diagnosed until she was 10.
I honestly sympathise, but this is how it is. Get all your evidence and fight. And be prepared to keep on fighting every time you need something else for your child. It's awful, but this is how it is. I was also the SEN school governor for 5 years, so I've seen it also from the school's side and their frustrations with the LA too.