Should i leave my druggie boyfriend?

when i met my boyfriend 2 years ago i knew he smoked weed and i had tried it before but i had never brought it or become addicted in any way. i never said anything as we had only just met it wasnt my place to. about 6 months into the relationship he said he wanted a baby and me being stupid agreed but on the condition he gave up weed. which i thought he did there was no signs of it i really believed he had stopped i got pregnant and had a beautiful baby boy, however wen my baby was a few weeks old i found out he was smoking weed not only that but that he had never stopped i was so hurt but he couldnt see why?!! we argue all the time about him smokin, we dont have much money and i feel that any money we do have could go on something better but he just says im a nag and want to tell him what to do? i dont feel thats right i feel like i want him to be a better person. he has a god job but i can see him going downhill. i told him i wont have any more babies with him or marry him (something he really wants) if he is still smoking weed, but i just cant see anything changing! am i wasting my time?