Hello K:
Well, I don't know if your question got answered. I don't think so, because we don't know what your lease says. But, it appears that your lease isn't CLEAR about this apartment...
And, I agree with you. It probably ISN'T a legal apartment...
So, the question comes down to what are you going to DO about it? You want to stay, and you want him to leave. You may not be able to get what you want, but I'd sure take a shot.
Sit down with him. Tell him the truth, that you don't want him living there. If he doesn't leave, you'll report him, and break your lease. I'd follow it up with a certified letter stating the same thing. IF the apartment ISN'T legal, he can't sue YOU for the balance of your lease.
excon