Christ. This is a tough situation. Step-parenting is a minefield and if more people knew what they were getting into, they probably wouldn't go on to have second families. (By that, I mean having half-siblings with another partner.)
"First family" children always feel conflicted when a parent - either mother or father - go on to have more children with another partner. They feel that they weren't good enough, and they feel betrayed by the parent (or parents). They feel unsafe in their status. It's like they were the "dry run/test" but it didn't work out so they were expendable.
So these awful feelings are not his fault. What he needs is a whole lot of support and, probably, counselling so he has an environment where he can say anything he wants to in a safe environment. He might want to call his parents every name under the sun, and moan about you, or his sisters. But that's okay.
There's a scared boy in there and his mum clearly isn't interested. He's 11, he's in senior school facing a heap of expectations, the world is a confusing mess, and he feels abandoned and resentful.
Throw into this mix social media, gaming, vile hardcore porn and gang violence. The internet porn issue in particular is highly toxic for boys. And now he's living in your house and even his father doesn't know what to do with him.
In my instance, I got sick to death of feeling like a fucking referee while my DH stuck his head in the sand and wouldn't engage with the problem. And is WAS a problem.
I think you and your DH need to sit down and actually talk through what this needs to look like. Does DSS have his own room? What are the house rules? Does his Dad make an effort to do things exclusively with his son? Going to the DIY shop or doing some sport together? Your stepson needs to feel reassured that he is his father's son, and that his father loves him just as much as his daughters with you.
It's really challenging to bring together a blended family successfully. Do please seek help and guidance.