Well, at the moment he's in school 2 hours, they're not teaching him anything and they are (illegally) taking the 121 money as if he were a full time kid while not delivering any of his EHCP interventions. So that's not great and I really am not getting good vibes about the school.
Yes, I know, schools are under a lot of pressure. I get that the TA is probably doing useful work elsewhere. But this is not good enough. It feels very much as though they are planning to babysit him until yr 1 and then morally blackmail you into homeschooling him. In a nice polite head-tilty sort of way, but still fucking you over and damaging his future.
Special schools are very hard to get into. And not everywhere has the full range of them.
Alongside figuring out whether that's the right option you should be figuring out how to make mainstream work (because you may well be stuck with mainstream) and simultaneously building the evidence base that could open up special school as an option.
Here's some ideas. Hopefully others will have more.
Claim DLA, if you don't already. Opens doors and money useful.
Offer to supply stuff that might help to deliver interventions - wobble cushions, fidgets, books on emotional regulation, whatever. Can be surprisingly effective at unlocking support and worth a go.
Get - in writing - from the school the hours they let your son attend now and the plan for ramping that up both in terms of hours in and in terms of delivering interventions. This does two things a) it means you have evidence that mainstream can't meet need if you need that evidence and b) it pins the weaselly bastards down - (have I mentioned that I am not getting good vibes here?).
Go and look at other mainstream state primaries. Especially at ones that are undersubscribed, have a bad ofsted or are in a 'rough" area. Tell them about your son and ask if he'd suit a place. Demographics mean a lot of schools are struggling to fill places and you might be pleasantly surprised. You don't need to tell current school you are doing this. Can EHCP cash travel with him?
Slightly more aggressive: Ask school - in writing - to itemise what EHCP money is being spent on and how it is supposed to benefit your child. Bring in SENCO as well and head. Be persistent. If email isn't answered follow up a few days later.
If nothing is improving you might want to start bringing the head of children's services at your council into the conversation and/or making a subject access request to find out what they have written down privately about your kid.