SO is TERRIBLE with finances!

Agh! My SO frustrates me SOOOOOOOOO much. On impulse, after I told him no, he went out and spent $350 on a ferret & supplies. He doesn't budget or keep track of his spending. He wants to get a credit card to "buy bubblegum once a month". I understand he wants to raise his credit score - but we recently agreed to trying a joint bank account for bills. I don't do credit cards, I believe in cash. He keeps saying, "you're not going to touch my credit card, you're not in control of finances, etc." when we share a car and we both work full-time, we spend money on each other all the time. It's "OUR" money. I told him I don't think he should get another credit card because he hasn't paid of his first one. He says he wants to pay bills with it, but that it's "pointless". My suggestion was pay off the bills with the card and use the bill money to pay the card. He argues with me about it and makes me feel inferior, like I have no clue what I'm talking about. Any suggestions?