Over spending ruining the relationship

Hi! So my fiance and I have been together 3 years, we have a 9 month old son together and I'm a second mom to his 5 year old daughter whom I've helped raise since she was 2. My fiance smokes a lot of weed, we live where it is legal and there are cannabis stores all around our city so its not a big deal to me as I occasionally indulge as well. However, my problem is that he is spending wayyyy too much money on the weed. I am currently a SAHM mom who is going to university as well, and has a side gig baking cakes/cupcakes to bring in a little extra cash when I can. My fiance is working construction as a carpentry apprentice, he's making good money but we are still barely making ends meet and are constantly late paying our rent. He got payed yesterday and has ready spend $100 on weed. We still have car insurance, rent and cell phone bills to pay. He won't listen to me when I try to talk to him about it, or will tell me he's going to slow down but then 2 days later he's back at it. He spends hundreds of dollars a week on weed and I'm done with it. I'm tired of stressing each week about whether or not we can pay our landlord, or feed our kids or even put enough gas in the car to get his daughter to school. Ive had to ask my parents far too many times to help me buy diapers for our son because my fiance prioritized getting high over our son. We even had to cancel our wedding and postpone it until next year because we couldn't afford it this year due to his spending habits. I just dont know what to do or say to get this to stop