If you were sure about this guy being marriage material, you wouldn’t care if he made a ring out of a paperclip. It looks to me like thinking about a ring is a great distraction from some much more important issues. He’s not a “bit” immature. He’s a lot immature. At 25, he’s still being irresponsible about such adult responsibilities as paying his bills and managing his credit. When he gets mad, he throws tantrums. He sounds like he’s about 14 years old and you are being put in a mother role. Please. You deserve far better.
Many couples take turns making the lion’s share of the money for the two of them so each can pursue school or some other goals. But it can only work when the person who is on the receiving end of the financial support is appreciative and shows it by doing as much as she or he can to contribute. That includes being careful with the money that does come in. Your guy doesn’t even manage paying his own bills on time!
However charming he is, I do suggest you take a big step back. Move out, stop paying his way, and let him fend for himself so he can experience the consequences of being so disinterested in how money comes and goes. Otherwise, how is he ever going to learn? If he cleans up his financial act, then you can enjoy his other qualities without putting your financial future on the line. Same thing goes for his temper. If he is only controlling himself because you’ve threatened to leave him. He has some personal work to do. He needs to learn self-control because that’s what adults do.
You aren’t being selfish to want him to man up. But I do think you are fooling yourself if you think he is mature enough to think about marrying.
I wish you well.