Does he see his father? What is the history with you and his dad splitting up?
I am in no way suggesting his behaviour is ok (I agree that removing the phone is a good thing to do, amazing how suddenly less crap something is when they dont have it!), but I am wondering if he is kicking out because of something else.
Not having a lot of money can be hard for kids, especially when they think their friends are rolling in it (or more likely, there parents are up to their arses in credit card debt). And if the split was bad, his dad was abusive and he doesnt see him anymore, or he does but dad slags you off, then he could be reacting to that.
My friends son was like this. He was fine, a great kid, until he saw his dad attack his mum (his dad had been careful until then to only have a go when the kids werent there). Mum and dad split up, dad slagged mum off every time they saw him, said that she was a terrible mother, that he was going for custody etc. In the end he lost interest when the next GF came along and her son went totally off the rails. He was parroting everything his dad had said about her. She got counselling and it was hard but he did come out the other side. He needed someone to blame for his dad being gone, for his dad being shite, and that was her.