You're going to have to tell him that you don't want to have any contact with him and that means no drugs and no sex. It won't be easy for you I'm sure but it you want to get clean and sober then this is where you have to start. Otherwise it will be you trying to hide from him and that's no way to live especially since he lives opposite of you on the same floor. Hiding is no way to live, there's no peace in that. You need to be comfortable where you live so ultimately you may have to move. Meetings would be very helpful now to help you keep your resolve about not relapsing. I wish you well.