I want to be ok with it... but I’m not..

My fiancé and I have been together for three years.. we have a beautiful 3.5 month old son together.. before we got together he was a very heavy smoker, we got together and he slowly but surely quit. For over a year and a half. Then, without my knowledge he started again. I’m not ok with smoking at all. I have my reasons and he knows them very well. I told him I didn’t care if he wanted to kill himself it was on him but once we found out I was pregnant I wouldn’t allow it around our baby. Ever. He promised he would stop. He quit again for a couple months, then again without my knowledge started again. We got into a huge fight about it when I was 7 months pregnant and I threw them at his head. (I’m bipolar and being pregnant meant I couldn’t take my medication so I was absolutely miserable and stressed beyond belief.) I missed anyway, but I didn’t want to miss. After that I knew it was a much bigger problem than I realized. He still smokes and our son is going on 4 months old. He says it’s because of stress but I don’t know what he could possibly be stressed about. All of our bills are paid, on time or early, we have jobs we like, our baby is probably the easiest baby ever. I don’t get it. I try to be ok. He never EVER does it around the house, car, or our son. Only at work on his ten minute breaks and lunches. But hearing him talk about smoking or how he finished his two packs he bought less than a week ago, makes my stomach turn. I want to vomit and cry and scream...... he doesn’t get it and neither do I....

Update. Since clearly everyone is missing the point here. I’m not trying to get him to quit anymore and I can totally tell he’s keeping it away from our son and “respecting my wishes” but that doesn’t change the fact that the thought of him smoking, of me losing him, makes me sick to my stomach. It isn’t the smell. It isn’t the price tag associated with it. Yes I’m harsh. Yes I told him if he wanted to kill himself it was on him. In my eyes that’s all cigarettes are. A death sentence. I fully understand his addiction is hard to break. I don’t rely on stuff to deal with stress so I will never comprehend the need to smoke. I’m ok with never understanding. We work together, I know his stress levels at work. He isn’t stressed about work. He won’t tell me what is stressing him besides “bills” but once again, I don’t care. I try to help his stress levels and I don’t talk to him about his smoking anymore. I seriously just came on the app to vent. I didn’t ask for advice. There isn’t anything that can help him until he himself wants to quit. I just don’t want to lose him. I’ve lost too many. I can’t lose him.