TUD, what a situation for you. So little support where you are and such a burden to have to carry.
I'm an adult on the autism spectrum and a mum to a disabled child, so I have some insight into this.
[just a bit of background info in case any people are thinking blimey, are all autistic people like this...Whilst a few individuals are violent, violence is not a feature of autism or part of its diagnostic list, so there is absolutely no reason why any person with autism has to use violence or should be allowed to endanger anyone. Some will lash out in toddler-panic-style without thinking, though, and a few are indeed deliberately violent, the same proportion as any other member of society might be. The majority of us are very peaceful people with a good grasp of social responsibility who do all we can to treat others with gentleness. I work with a huge number of men and women on all parts of the autism spectrum who are vicars/church helpers, accountants, artists, poets, writers, speakers, teachers, surveyors and every manner of volunteer and helper, and who have found ways to contribute to society no matter what their IQ (some have profound autism and very low IQ), so it can be done with the right support and information.]
Your support has been missing so it's no wonder you're all struggling. It's in no way your fault.
A friend of mine was faced with this situation with her adult daughter who has a variety of disabilities including autism, and after taking a few attacks, contacted the local police and explained the situation. They came round to talk to the dd and explained that hitting someone is against the law and if she did it again, she might be put in prison. For her, it worked beautifully. Other solutions may work for other people. The police can be a good ally. There are certainly residential placements out there, and social services have a duty of care to your son and to you. I would write to them to say what the situation is, that there is real physical danger, and that you need them to assess him immediately as you are not able to cope any more because of the disabilities in the rest of the family. Sounds dramatic, but unless they have those words, they may rate it as not worth bothering with. Tell them the Autism Act laws means they absolutely must offer assistance now, which is true - they must.
In a meltdown situation, don't engage with him other than to try to ensure his or your safety. no speaking unless there's no choice, no eye contact. Minimise all and every interaction and all the sensory overload that he might be experiencing, so he has a chance to calm down. The advice to take it in turns if you can is a good one. If you can record an incident, take photos of bruises or damage etc to add to the evidence, good. It's not to hurt him, it's to get the help he and you need, because no way can he contribute to society and feel like he's making a difference if the best he's doing is beating up the people who love him.
Hope it helps a bit...
(PS I'm having treatment for cancer at the moment - really hope you get good news about that...)