It sounds like you need to sit down and have a long heartfelt chat with your DH. I second counseling too and please tell centerlink. It's easy to change if back if he moves back with his mum. If she gets a bee in her bonnet, it's just how it is, he's living with you, you've got the extra expense so you need the money.
Other than that, I can only tell you my experience of having my half sisters and bro stay with us. All children are equal in the family, all have the same responsibilities and rules, it was the only way to keep the peace in our house. Just because your DSS is new to living with the family doesn't mean he is not got the same rules as or more rights than the other kids, albeit he may need time to adjust but beyond that i would endeavor to treat him the same.

Sent from my HTC Desire using Tapatalk