What I would do (and don't consider this advice, because I don't pretend to say that this is the BEST course of action, just what my lazy-a$$ would do).
Nothing.
I would continue to brew, and just be a little discrete when you're carrying precarious looking equipment in and out of the apartment. If you have the opportunity to talk to him about it, do so, but I personally wouldn't go out of my way. You know it's not dangerous, and you know you're not being irresponsible. I live in a shared living environment to, with condos next to and below me. My biggest concern is bottle bombs, and spilling 5 gallons of wort/beer somewhere, because I fear it would drip down in to my neighbor's place. So I'm extra careful in those respects, and try to avoid using carboys (unless my pipeline is full).
Plus, I'm no legal expert, but I'm going to GUESS that until he issues you a request to stop in writing, there is no legal recourse he can take. If he does do that, then maybe you need to explore a few more proactive measures (see all entries above).
Good luck!