Linda,
Can you pull one of your credit reports (if you have not already) and verify that you are listed as joint and not an authorized user.
www.annualcreditreport.com is free.
If he just called and added you one day, my suspicion is that you are just an AU. As such, you can charge all you want, but have zero liability to pay.
If it is joint, you could be liable if he stops paying (or passes away). Normally, you would have to be involved in the application process. If it is joint, write them and ask for a copy of the application you signed when you agreed to be responsible for the bill. That should get them thinking....
Keep us up to date on how it goes.
LG