Honestly, you don't have any worthwhile options. At this point you've just got to hope and pray that he actually completes the loan and you don't end up toting the rest of the note.
If he can't refinance then there isn't anything else you can do because assuming the loan yourself and just paying it off and have him pay you but I seriously doubt an option of such would even remotely be on the table considering he's already demonstrated a history of not paying the loan on time.
I won't bother lecturing you on why co-signing (while not always a bad thing to do) can be a costly mistake as I'm sure you're aware and is currently learning the hard way.
OP, I hope that the relative continues to at least make the payments so you don't get totally screwed in the end but the only way to stop the late payments is for you to always keep the loan up to date yourself...and that means you burning money out of your own pocket. At this point you have to decide what's in your best interest...to lose money or take the credit hit for the continuous late payments. Sorry...I wish I had a better answer for you but this is a clear case of how sometimes the good guy doesn't prosper.
You really have 3 choices
1. Continue as is and hope it changes (whats the definition of insanity?)
2. Make the payments to the bank yourself and hope he pays you
3. Take the car and sell it