I worry about how I’d manage to work if DH wasn’t around. I work part time and he looks after the children when I’m in work.
You can not leave this man with your vulnerable child alone. He abuses him right in front of you, what do you think he's like when you're not there?
I have a non-verbal seven year old autistic child. For reference, his father and I have NEVER raised our voice in anger at him, or said anything cruel to him EVER. It isn't normal to abuse or lose your temper with somebody so vulnerable and in need of protection. The mental health issues are an excuse. I've experienced multiple serious mental health issues and never abused my kids. Your dh is simply cruel. It isn't a trait that can be counselled or medicated 'out' of someone; he'll never change.
Call social services and tell them that your child is being abused by his father and request their advice on next steps. This will ensure that it is logged and help you stay accountable when things are difficult.
Next time he leaves the house, change the locks and message him telling him that he can't come back and that you will call the police if he attempts to return.
Ensure that every message you send focuses only on his abuse of your son and your intention to protect him. He's likely to twist your break up to suit himself, but this will be harder to support (especially in family court) if you are very clear about your position and don't end up 'kitchen sinking'.
He'll probably try to get you to feel sorry for him. Always keep in mind that your loyalty and responsibility lies only with your son, who is incredibly vulnerable and only has you to protect him. Your dh is his own responsibility. He's let you all down big time. You owe him nothing.
As far as work: quit. Your child is more important than the job. Apply for DLA. You don't need a diagnosis. There are websites online, e.g. Fight Back for Justice, that will help you fill in the forms. You can look on the entitledto website to get an idea of where each possible rate will leave you financially. If you're not an especially high earner, you're very unlikely to be worse off.