Living like that is a health/safety hazard for you and your girl. He attracts bugs and could bring pestilence to your abode.

I would have a serious assed talk with that guy, remind him who is the owner here and that his nasty habits are going to change.

If it doesn't work, stay home from work one day, toss all his shit out onto the front lawn and change the locks.
If he gives you any shit then beat him with a stick (just kidding...don't beat him with a stick).

It sounds like you would have to have him out of there to get a new roomie anyways.

There is no reason that you should have to be putting up with such filth.

I would say to get him a girlfriend. But, who would have him?