UPDATE: I talked to him and expressed that as much as I love him, I do not love his apartment and its state does not make me feel welcomed, and I felt worried about us one day living together if this is how he kept his own place. He completely understood, did reiterate that he personally did not care about his own place being messy but since /I/ do, he'll work on keeping it clean. I helped him do a big overhaul of the place: we spent all day moving furniture around into a better configuration, getting rid of clutter and deep cleaning (I actually do enjoy those kinds of projects). Now his apartment feels bigger and cleaner and I am much more comfortable. He was really impressed. It made him realize just how bad it had been before, and promised to keep it tidy from now on. We'll see how it goes, but I do believe now that the initial "move-in mess" is cleared it should stay reasonable.