How do you tell your partner you dont like them smoking?

If your objection is coming as a surprise, introduce it gently. Dont say, Stop now, or Im leaving, but rather, Id like to talk about your habit. Can we find a way forward here? Then try to explain your feelings such as, When you smoke, its like youre not really in the room; I feel ignored.

Is it controlling to not want your partner to smoke?

No, you are not being controlling. You just want whats best for your health and his. Your reasons for not wanting your boyfriend to smoke are not controlling. An addiction to nicotine is difficult to overcome, however, a person really has to want to stop smoking in the first place.