get 2 ounces of pipe tobacco, soak this in a jam jar for 3 days, take the tobacco out and squeeze it so as much of the liquid goes back into the jar as possible, boil the liquid on a low heat til all that is left is a viscous sticky yellow substance, this is pure nicotone, 3/4 drops in a coffee (to cover the taste,) will kill him, problem sorted!
erm..........

On a serious note for the OP. I think a good (aka nice) approach might be get some of the other house mates to talk to him about it also.
This may have already been done but its not been mentioned.
My reasoning is that if its you that is constantly "bugging" he may feel that it is only you that has the problem and that the other house mates are fine with him.