Partner spends every penny

OMG... I knew he liked to buy stuff but it's getting worse.

He's always been a spender which was fine before we had a baby. During my pregnancy I suggested we join our finances so I can have better control over our household budget.

We now have an 11 week old baby and he seems to be getting worse. I decided a few weeks ago to only use cash for our weekly budget to avoid over spending - Christmas being round the corner and between us we have 3 other children plus he pays child support for his boys.

I ask him where he put the remaining budget for this week because we need some bits and it's gone, all of it! I ask how we spent £150 in 4 days, he just said he doesn't know. We haven't really done any food shopping because we did a big shop last week, the car is near full, I prebuy everything for the baby in bulk, my daughter isn't at school this week so there are no school expenses and she has a job to buy what she wants outside of essentials and I barely leave the house (it's cold, I'm not a winter person but I do go for a daily walk with the baby...which is free).

So if anybody should no what has happened its him but he doesn't know.

I'm worried! I spent 10 years as a single parent to 1 child and I always made sure I maintained a stable home for my eldest, now I have 2 children and I don't feel financial stable anymore (not because of my girls but because of my partner). It's very easy to say that I should have considered this before we had a baby but I had it under control before. My personal income has dropped because I'm on mat leave, I don't have the time to manage every penny like before, he was never this out of control before and we have discussed this. We were so stable for so long that we decided to increase the child support payment we make to the mother of his boys (I say 'we' only because our finances are joint, I am not obligated to pay his child support).

I can't live like this, I refuse. I like stability but what do I do? How do I make him understand? How do I make him try to be more responsible?

Am I crazy? This morning he acted like I was over reacting.