I know what you're going through. I rented a house with my sister. I had quit smoking for about four years. But she smoked. At first I was okay but within a few months I had started borrowing a cigarette here and there. It wasn't long before I was back to half a pack a day. We decided to quit together. That's the only way I was able to quit again. Now I'm smoke free for a year and a half and she has since moved away.
I wish I could tell you a trick or secret to help you. The only I can think of is to try not to be in the same room as him when he smokes.
Also, it helps to quit if you know what compels the addiction. Is it just the nicotine? If so, you could try the patch. (I've never used one so I don't know how well it works.)
Or is it an oral fixation? You could try sugarless gum or healthy snacks like carrots and celery munching to help you with this.
Or is it that you need something in your hands all the time. You could get a stress ball to play with...
Good luck.