I agree with what Susan said mostly. My input:
Clean up after yourself (dirty cups go in the sink, not left around the apartment);
Don’t eat the last of anything unless you know ahead of time it’s okay. This includes not eating food that was set aside for some other purpsoe (tomorrow night’s dinner or whatever);
When you guys go out, it’s not an automatic invitation for him to come along (although you may want to invite him!);
Don’t bring strangers over to the hosue when you’re not there. Friends of friends of their friends can sometimes turn out to be people you really don’t want in your place.
Have him be actively engaged in some pursuit of happiness that gets him out of the house once in awhile. There’s nothing more depressing than coming hom to someone who’s been laying around the house doing nothing all day. Especially a small house.
Wow. Our place in California is 1050 sq ft for two adults and two kids (and 2 bedrooms) and we’re cramped beyond belief. (Of course, part of this may be that my husband has taken over the entirety of the “dining area” as his home recording and music studio!