What should i do?

My fiance smokes weed and has done so for pretty much his whole life. He told me about it after we started going out and naturally i was a bit dissapointed to hear that but i accepted it because i had feelings for him. Wheni became pregnant with his child my feelings on it also changed.He promised me he would quit and he let me throw out his pipe and his weed. He was doing good but every now and then he smokes it. This really bothers me to the point that i want to call our wedding off until he decides he is going to take this seriously. I'm really tired of going back and forth and having him tell me he will change and not do so. I am always paranoid when he goes outside because i think he is smoking weed. Its affecting my relationship with him, though he cant see it. What should i do? We also have a 3 month old baby and i dont want her to know her daddy smokes weed.