OkamiKodomo, it's also not just about the stress and inconvenience. If he's smoking pot in your house, he is making you criminally liable if he get caught. And from what you've said, he doesn't seem responsible enough to keep his act together and not get caught. If the police raid your place, you could lose it (it's an apt, right?) and you very well could lose that great (though stressful) new job of yours. All because some freeloading friend doesn't want to get his own act together and gets you in trouble for something they did.
Stop thinking about him - he certainly isn't thinking about you. Point him to the door and then talk to your fiance about it.