Well, for starters, have you made it clear to this guy that you and your ex are definitely over and are only together because of the house situation?
How long have you been living together but not together, if you know what I mean? And are you looking to settle this permanently? Because one of you will have to move out pretty soon if you want to move on into other relationships.
It seems pretty obvious to me that this guy feels really uncomfortable with your living situation, so you need to reassure him that you like him and you want this to go somewhere.
You obviously can't bring him back to yours (or at least I don't think so, not while your ex is still living there). Are you ready for that stage yet? Because if so, invite yourself round to his for a 'sleepover'. You can't get more obvious than that!