It sounds like its not quite to the point where it needs to escalate to the steps you're talking about in terms of buying him out, and this could be solved with a quick 15-minute company meeting about use of the company card/funds. Maybe there's something deeper there like he's having financial trouble that needs to be addressed. Or maybe he's just really bad with money, in which case maybe you should suggest someone else hold onto the company card.
I think as a whole, our culture is too quick to jump to getting lawyers involved, when you should at least try giving just talking to him a second shot. Even if you get lawyers involved and take care of taking him out of the company cleanly from a legal point of view, it's definitely going to be a HUGE distraction and morale killer to have a founder leave.