Jem, as you've said you're both young, but you're both still adults. You know what? I'm thinking this is his first serious, long term relationship, right? He's still living with his parent (s) and he's pretty much always had everything done for him....he's never HAD to take responsibility for himself because there's always been someone there to mop up if need be.
Rather than just ditching a relationship you don't want to ditch, could you put plans to move in together on hold for a while? Is it possible for you to get a place on your own without him? He really needs to take a step into the real world before anything will really change. I'm remembering when I first moved out of my parents, I didn't stop to think about the fact I'd have to stop buying (as much!) clothes in order to pay the bills or get food in for myself, but I sure got a wake up call pretty quickly, maybe that's what he needs more than anything.