I definitely wouldn't be able to turn a friend of mine down if they legitamaely needed help. I'd probably give him a deadline though. Like, "You can stay with me until February _____ but by then you need to have either found a new place to stay/have found a job or whatever. Then you won't have to "kick him out" because he'll know in advance how long he is welcome.
Yeah, that sounds like a no then. I'd just be honest and tell him that you can barely afford to feed yourself/your girlfriend, and that you wish you could help, but money won't allow it.