This guy has a job, pays the bills and smokes once or twice a week? This guy doesn't have a drug problem, he has women problem. Your just being controlling, and changing goal posts left right and centre. Him smoking really does not impact you or your career. Especially if he is only smoking as much as you say. I know several teachers who partake in smoking weed and other drugs. Your private life is your private life, people only know what you tell them, nad in any circumatance drawing a line between your work life and persoanl life is a must. You sound really selfish, do you not think you being a skint student reliant on him has had an impact on his options? Your worried he might kick you out, so you should. Stick your ideals up your arse because in the real world its about being realistic, He hasn't given up because he enjoys it and probably feels he has got the balance right. He probably won't give up any time soon. You don't have to be with him, you don't have to date a smoker. Make a decision because he has made his, he is still smoking. He probly doesn't take your threat/ ultimatim seriously because he quite frankly thinks its petty. I don't blame him, and he's probably thinking if your willing to leave him over that then just ****ing do it.
The only person that can **** up your career is you, and thats the truth. Unless this guy is a useless boyfriend beside this, than i think you should put things into perspective, i wouldn't be surprised if he feels completely deflated that despite all he does and has done, him smoking overrides all of that.
This genuinely comes across you care more about how this makes you look than the speculative health risks.