I second everything Ellie56 says.
Your poor Ds! There's a lot to unpick here. The first thing you need to do is to read up on the guidance/law for schools around supporting children with additional needs. This is a good place to start:
www.ipsea.org.uk/how-your-nursery-school-or-college-should-help
His school can't simply do nothing if he's struggling like this. You need to really push this. You may have to become 'that parent'. Email the Senco, put everything in writing so there's a paper trial. Ask for a meeting to discuss what they're going to put in place to support him. If this isn't an independent school, they will be getting funding to support children with SEN. By their own admission, he is already two years below expected levels. (Although that funding won't be ring-fenced for your Ds without an EHCP, it still exists.) Ask in writing what the plan is to help him catch up with his peers and support his barriers to learning.
Find the school's SEN policy - it should be on their website. Or you can ask the Senco for a copy. When my Ds was at primary he was on the SEN register and I had a termly meeting with his teacher to set and review individual targets.
Tell the Senco you would like your Ds to see the school's EP and Speech and Language specialist. (They may try and fob you off though.)
If the school won't work with you to help your Ds, then you can apply for an EHCP assessment yourself, as Ellie56 has said.
You don't need any formal/legal training to do this. I did it myself when my Ds's school wouldn't support me.
As part of the assessment, your Ds will be seen by a range of specialists, eg an educational psychologist, speech and language, an occupational therapist. Their reports will/should set out what your Ds's barriers to learning are and what needs to be done to support him.
Does your Ds have a diagnosis of any condition? If sounds like you have some concerns around some aspects of his development. Have you spoken to his GP?
In terms of your living situation. You don't say if you rent or own. Are you keeping a diary of your neighbour's threats? Have you spoken to the police/the council?
It sounds like things are very tough for you both. With the right support in place though, it can make such a difference to your Ds. I'm afraid you may have to be prepared to fight for it though. Don't take no for an answer.
There are very helpful, knowledgeable people on this board who are invaluable sources of advice and information so do post back if you have specific queries about the process.