Oh gosh that sounds so difficult, for him and also for all of the family, particularly you as you are trying to spin all the plates and help everyone as much as possible. It's wonderful your eldest is loving and tolerant. Does your LA have a young carers scheme? They organise events and support for children with disabled siblings. As my older is the child with additional needs (that we know, my younger son is still very young) this isn't relevant for us right now but as my little one gets older I will look into it for support for him from a group who 'get it'.
My son has always been non-verbal, I suppose pre-verbal for a time (some professionals still say this as I suppose it's more positive) as he used to have single words for a very restricted set of requests (mostly food and drink which is common as they're very motivational!). He now has no words, only sounds which are mostly loud noises, groans etc. If he does try to repeat after you it is unintelligible now, just a random noise. He used to play with a small range of toys, cars, play doh etc somewhat normally. He has never had imaginative play ability but would vroom cars, make play doh snakes etc. He now throws toys as he likes the sensory input from the sound and crash (I think) and play doh too. He has become so difficult with transitions that going anywhere is a nightmare and he tends to lie in the bushes when getting off the taxi at school as he doesn't want to go in. It usually takes a team of three quite some time to coax him in or however they manage it. His self-care has regressed, he has never done anything completely independently but we used to back chain hygiene and dressing and he'd do the final bit but now he's unwilling or unable or a combo of both. He doesn't sleep until after midnight so we do a day from 6/7am until he finally passes out after midnight in which he is either doing nothing or being destructive or melting down in a really distressing and loud way.
I am not trying to scare you. My son had peers who presented similarly when younger who have calmed down, accessed therapies and services which have helped them progress and be happy. My son is really complex and it is really hard all round, for him and those around them.
Knock on all the doors: OT (home adaptations and functional/sensory), CYPS and or CAHMS, keep on at your GP about both your son's needs and difficulties and your own stress and distress at this. I've forgotten if he's under the community paediatrician still? If so, email or call their secretary asking for help. Involve your local councillor and even MP if nobody bothers with you. My local councillor helped us get a disability social worker after the first one assessed him as being 'able to access universal services' as I told her I drove 30 miles each Sunday to take him to a SEND softplay session. Work that out!
I am here any time. Don't let things snowball any more than they have already, you are doing a brilliant job and the job of many which they'll try to leave you doing because you happen to be mum. If services and schools etc reject children with needs like ours because they can't meet need then how exactly are parents who are ordinary folk supposed to meet those needs and sometimes on a 24/7 basis while being an emotional and sometimes literal punching bag!? That's my question! He's your son and you love him but he and you need the support of others to care for him and keep you sanity and health. The mythical village would be amazing but as it is, many of us have to fight tooth and nail for services to be that village for us - or at least as close as we can get.