boyfriend goes behind my back with weed

I knew going into the relationship my boyfriend smoked but I made it clear how I felt. He stopped doing it for a while. Then since September weed has been an issue four times. I got to the point where I have said if your going to do it let me know. I don't want to just find it and be thrown off well after it nearly breaking us up because my dad found blunt wrappers in his car he swore he would never touch it again, that he hates it because he almost lost something important and so on. Well last night I found a rolled blunt on his phone from where he had smoked the day before. I've made it clear how I feel and to atleast tell me. I feel like ALL trust is gone I mean not one time has he told me about it I've always found out. I'm trying to forgive him one last time because he actually cried on me last night because he knew how serious I was about ending it. I do not want to end it but my mind is weighted right now and it's hard to be happy when I feel betrayed. How do I move on from this and is there any reason why he can't move on from the weed like mentally maybe it's an obsticle and if so how do I help him overcome it