How about bringing up the car payment with something like, "Hey, I know you probably haven't forgotten, but just in case, I wanted to remind you that the car payment is due on ____. You're taking care of it, right? Cool, thanks! Just wanted to be sure. If something comes up, please let me know ASAP so we can handle it immediately. Thanks again!"

If he doesn't pay it on his own and you end up having to make the payment again, then you SHOULD put your foot down. I know it will be difficult, but I think you know you've got to do what's best for the both of you. \:\)

Getting over a painful experience is much like crossing monkey bars. You have to let go at some point in order to move forward. ~ Joseph Campbell