Not clear where you are located, but in USA there are a large number of states (though not all) where both spouses are legally responsible for the debts contracted by either spouse after the marriage. Keeping a separate checking account, refusing to co-sign for him etc would probably not protect you from liability for his debts in those states. You need to contact a lawyer in your state (or province, country etc if you’re not in USA) to get the full, correct details. Even if you end up in some jurisdiction where you aren’t responsible for his debt, if he should spend all his money foolishly then you will be stuck with the responsibilities of paying the rent, utilities, grocery bill, expenses for your children’s needs etc out of your own money.
I agree that unless you both have a plan in place to prevent this man from foolishly spending money that should be used on couple/ family necessities, such as having his contribution to bills deducted from his pay before he gets any, cutting up his credit cards, and making sure he isn’t going to take your money or credit cards when he’s in the throes of a bipolar spending spree, you should not marry him.