My fiance goes through cycles of weed addiction and I don't know what to do about it.

My fiance has been on again off again with weed for our whole relationship. I don't mind him casually smoking every once and a while but I really don't like when it becomes everyday and eventually all day. Ive told him countless times how I feel. Usually it gets to a point I blow up and he quits for a while before he starts casually again and the cycle repeats. What bothers me the most is when starts driving high. I think it's unacceptable and I honestly don't know what to do at this point, how to help or if I should just end our relationship. He is an amazing guy otherwise and is so good to me but I hate this cycle. What do I do? Can he smoke weed casually or should he just stop forever?