Goodness me I understand how frustrating it is when you find out information after the fact.
There was lots of dodgy dealings with foster carers with our eldest, and we had to fight to get all the information after they had been placed. The anger is real, and I have spent many nights lying there with my boy in my arms the way his foster carer should have done and didn't. Fed him the way they should have done and they didn't. Taken care of his body and hair the way they should have done and they didn't. I'm furious on his behalf and have cried when I think of his little body not having it's needs met the way he deserved. I'm also sad and angry for us because it took so much energy away from when we should have been bonding with our child and we were trying to work out what on earth was happening and why. I cringe when I think of all the time I wasted trying to parent in a way that wasn't very effective, and had we known all the history, we would have done it differently. It makes me bitterly sad.
If he is still there, don't accept nursery accepting delays. I know it's easier said than done, but get them to be specific about the interventions they are running to bring him up to speed. He deserves it. They should understand his risk factors are huge, and he needed people to take that seriously. I would be sitting down with whatever report they have been given, asking for a meeting with the new school and saying that you want him to either be making progress before Christmas or them to start working on finding out the reason why.
It is really frustrating when they don't see what happens at home. Our eldest is a dream at nursery 98% of the time, and it is only after nearly 18 months that the mask starts to slip at nursery every now and then. However they have seen the pick ups that are difficult, and we have been very open and honest with them about what we face at home. That really helped them to contextualise the behaviour we saw and how it had an impact at nursery.
Hopefully school will be more supportive because they will have more skin in the game- after all they will have him for 7 years, so it is worth it for them to get it sorted sooner rather than later. However it might help to speak with the senco as soon as term starts outlining your concerns and explain what you see at home and what your suspicions are, as well as where you are at with any referrals to community paediatrician, ASF etc.
You can definitely do a parent EHCP application if they aren't willing to assess though. Our borough was surprisingly helpful with this, and although it was laborious, it got the attention of the settings and the pressure of an external panel asking for information meant that interventions were delivered, reviewed and adhered to, which made a difference to our son. It is about your child's ability to access all aspects of life, social life and family life as well. My children cannot access that in the same way and it is detrimental to their health and ability to access education, and I would argue that if your child is already not meeting his expected development goals, there are grounds to say that it is affecting not only these aspects but his educational abilities as well.
Finally, do take some time to reflect and regroup. Living with and loving a kid who has multiple needs can be tricky and draining. (Being that kid isn't a basket of apples either, but this is Mumsnet not kidsnet so you get the sympathy here!) if you don't have someone to back you up it can be hard, and it is valid for you to feel all sorts of emotions about that all at the same time!