If your husband went to the doctor, and you didn't pay the copay, you are responsible for paying it. Legally, I don't know. Ethically, yes you should pay for the services you received.
Call and ask for an itemized bill, and ask why you never were billed before. But right is right, and if he received the service and didn't pay, he should pay now. Regardless of how long its been.
I would call that doctor's office and try to get more information. If they still want you to pay, there is a great legal forum on the internet... since we can't give links, then you can find it by using a search engine and typing in "expert law" + "legal forum".
Good luck !