HEllo dear lady. I have worked DD children, including those with Autism. However, I do not think one of your problems is do to your son being Austistic, the problem of him not sleeping alone.
As with any child without a disability, most children get their way by doing particular things. your son- I am guessing- throws a fit when you leave the room, or will not sleep with him. Am I right? Crying, yelling, carrying on...
The simple fact of the matter is, you will have to make your child sleep in his own bed. this is done slowly in steps. Look to the Super Nanny, or Nanny 911 website for mroe information.
As far as you being over-worked, have you thought of a sitter/nanny? Your daughter could probably help the sitter with the sitting, no?
Now I am guessing you are in the UK? In the states you can get respite help and a break from your care-taking duties; surely there are programs like that in your community.
Finally, in closing, I hope I am not being too forward. Do not feel bad that you have to discipline your son by making him sleep in his own bed. Do not feel bad about having your son stay off with another person. he is far stronger- and smarter in a manipulative manner in which all children are- than you think.
Take care my dear, I hope it works out.