I agree that you need to move out and that you shouldn't be around that stuff, BUT...

Unless you're actually partaking you won't test positive. Drug test "positive thresholds" are designed to not show a positive result with only a slight amount of drug in your body. For THC, the normal threshold is 50 ng/ml concentration in urine, and unless you've been sitting in a closed up closet for hours with him before the test I don't think you have anything to worry about. How do I know this? I had a roommate who smoked pot at one point and was VERY paranoid myself. Did a lot of reading.

But, when it comes to your career don't even take the slightest risk with this. Get out of there long before you need to take any tests.