I'm with Croz on this one. There are other issues in your relationship that you need to adjust. Money is just a symptom. you probably need to discuss your goals and see if they mesh, especially since you are thinking about marrying the lad. getting married will not resolve money problems, but rather money will become the issue.
financial management isn't a function of a person's intelligence, anyone can budget and stick to a budget if they wanted to do so. it is a matter of behavior.
most people are lazy and do not want to actively manage their finances. that is why pay yourself first is such a catch phrase in personal financing. if you set up some kind of transfer or savings automatically, you don't notice it. it's no surprise that he doesn't have issues with things being taken out automatically, because it is done before the money hits his account. he then has a fixed amount that he knows he can spend.
if his credit is bad, then DO NOT under any circumstances cosign for anything. i wouldn't even have a joint account with the lad. i will also second about not bailing him out.