Hi chic, you may have to accept that whatever happens, he won't get treatment. Turning off the internet may force him into it, but I think you should do it regardless. He's a very frightened boy right now, and he's dug himself a trench. Pushing him further on the treatment might make him dig in. The only reason he'll have to emerge is if his life gets so uncomfortable he has no choice.
Sometimes when we lose control on an argument we resort to threats. Children (and I count him as one) know once we start threatening we've lost already. You've given him the consequences of his action, now without further discussion it's time to go through with it. That's the only thing that will count for him. He may not go to treatment, or he may do it to 'trick' you into turning it back on, but you have to follow through quietly and resolutely.
The idea is that if he gets a job he will be able to pay for his own internet, right? Leave it at that.