Tee99,
my heart goes out to you.
You've been through some awful things and coped admirably all things considered.
Firstly, congratulations on leaving your abusive partner. That takes immense strength and personal resolve. So hats off to you.
Secondly, well done for resolving the educational challenges your son faced - to see him content, stable and flourishing - bringing home trophies - is amazing considering the trauma that he and your family had experienced.
Which makes it all the more bittersweet I suppose that your abusive partner has found a way back into his life despite your hard work to move your family on.
The ASD and ADHD bring an added dimension to this and your son is extra-vulnerable because he's living with a man who doesn't necessarily understand what he's been through, the progress he's made, and how best to support his development.
On top of that, he has a history of violence.
Your son is also displaying violent behaviours which might be a symptom of ASD + puberty hormones? or perhaps just a burgeoning new awareness of his 'difference' (loath to use the word) so all things considered, it wouldn't strike many people as a sensible match of parenting and child needs.
What's missing from your story is the support networks that you absolutely need to access right now. Here are a few suggestions:
Social Services childrens' team: tell them that you have a safeguarding concern for your child - explain what's going on, that you're worried about your son's welfare and development, and that you want him back with you, but need support to be able to manage this new behaviour he's presenting. He might need some specialist input from professionals, but there's no reason to assume he can't come over the feelings that are driving the aggression in the future given some time. People have such a negative view of social services but they always want to keep families together and supported if they can. If your partner has a history of abuse and can't demonstrate that he is aware of, and able to meet your son's needs, then that's a concern for them.
Gingerbread: charity for single parents - call them! see what they suggest
National Autistic Society - speak to them about the new behaviour he's presenting and what they'd suggest you do to get some help to manage it
Domestic Violence charities - there are a few, and often there are support projects locally. Not all of them focus on the act of leaving - some are there to ensure you can stay safe in the long term. one example is DART by the NSPCC (www.nspcc.org.uk/services-and-resources/services-for-children-and-families/dart-domestic-abuse-recovering-together/)
most important of all: you are not alone, there is hope and there is help.
take care
TAISN x