Trigger warning! Domestic violence

NAS88278 said:

the support worker has said she can't do anymore with regards to investigating domestic abuse because it hasn't come from the child. This is a child who can not vocalise his feelings and does not understand emotions

Does the child have a therapist helping them with their autism?

If so then I would ask the therapist to try to establish how they can build a case to take to social services that explains to them that their default approach is worthless for people who cannot vocalise like others.

Without a diagnosis this will be hard to prove so you may need to see if you can get this through private channels.

With your son being so conditioned it will be incredibly hard to undo this - your main weapon is being a good step-parent and provide them with the safety and respect they deserve.

Long term therapy will likely be needed to deal with the abuse issues here and it unfortunately looks like you won't be able to do anything through the normal channels. If you know some shady types who may help make the father less present then this is an alternative, but not one I could publically condone of course.

NAS88278 said:

How can I explain to him that what his father is doing to him (not physically but mentally and financially) is wrong and he needs to speak up and say it.

He will be going from one father figure telling him what to do - to another telling him the other guy is bad. If you try this approach he will likely shut down and block you as he doesn't know who to trust.

Offering a safe space and a willing ear is most likely to get results, but the therapy is where the real healing will be done.

I've been through abuse as well, and I've had issues vocalizing myself too, and I'm trying to think of what would be helpful. 

I would just suggest that you say to your step child that they'll always have a home with you, that they're not going anywhere, and that they'll still have all of their personal belongings with them. They're not going to be in trouble or be the bad guy, for saying things about the bad things that their father has done to them. 

I mean, if anyone tells a child not to speak up about the bad things that's happening to them, especially telling them not to go to the professionals or the authorities about it, then those people are likely not good people, because no good person needs to ever hide their good deeds from others. It's only bad deeds that are ever hidden. 

I mean it's hard to raise a child, because if you spoil a child too much, they can be spoiled rotten. But if you punish a child too much, they can get traumatized. So there's rewards for good behavior, discipline for bad behavior, and also education for their growing independence. But if there's only ever punishment and negative reprocussions for the child, and never anything else, then that's obviously abuse at that point. 

I hope that your step child gains the courage to speak up about the situation one day.

It's brave of you to reach out for advice. Going through the special guardianship route for a stepchild in a domestic abuse setting can be incredibly challenging. It's important to seek support and guidance from a professional who understands the legal complexities involved.