Smash his phone and cut up all his clothes
WTF? Hope that's a joke. 
OP, this is obviously a challenging time for many and I feel for you. I think it's important to look at the positives in this situation as there are some.
He's indoors and you know where he is. He isn't engaging in risky behaviour and isn't into drugs, drink or criminal activity. It also sounds like as long as he gets what he wants, his behaviour is actually ok. Since all he seems to want us being around his GF, things could be a lot worse. He's also not with bored mates looking for trouble.
Also, in terms of your home situation, it sounds like you've got heaps to deal with in terms of supporting your other DC/family atm, so if your DS is elsewhere and safe, at least he's not causing trouble under your feet. Especially since an unhappy DS sounds like a dangerous DS in terms of your other DC.
I know it's not an ideal situation by any stretch of the imagination. But it also could be a lot worse and I think it's easy to forget that when things get very stressful and you're not getting the support you clearly need with him.
My advice would be to help him repair the relationship with the GF's mother (by advising him, not by dealing with her direct- help him to sort out his own mess). And ask the PRU for support. If they're not open over Easter, call social services again if you need more support. And then obviously 999 in an emergency if he attacks anyone.