Ok. I'm going to be honest. Have you seen my thread called 'kids' ? My ex lied to me, hid his addiction and made a 'plan' to catch me which involved us having a baby. I know this because he told me afterwards. So we have an almost 2 year old, he's been around for maybe 1/4 of our life. Check the thread out anyway it's a snapshot of life with an abusive addict.
You CAN'T fix his addiction. Give it up now before it destroys you. You cannot do anything about it. Having a baby to him won't fix it, and a joint bank account certainly won't fix it. I can't even count how much money he's stolen from me without a joint account.
Why would he even ask that? There's no need to have a joint account unless he wants to control your spending or steal your money. Think about that, it might help you remember what kind of a man he is.
Addicts hit their bottom by losing people, among other things. I went to an open NA meeting and the one thing that was clear was that addicts didn't hit bottom by getting support from their loved ones.