I just wondered why you agreed to give him groceries. This joint planning of finances confuses me - once STBXAH and I separated he dealt with his bills/finances and I dealt with mine. Made my life a whole lot simpler. There again, we didn't have kids, just cats!! It still sounds as if you are managing his finances for him or at the very least trying to minimise his financial consequences.
I have no idea what to do in your situation right now though. You're angry because you feel he's taking advantage. Did you think he would stop drinking because he has so little money when you offered to give him groceries? What are your motivations for not giving him the groceries you promised? Teach him a lesson? Payback? Stop enabling? What would the consequences be for you and/or your children? You are allowed to change your mind! We codies are very good at reacting - we need to remember to take a breath, step back and analyse the situation while being self aware. I still haven't worked out how to do this automatically!!