In your case, primary evidence that your husband is a permanent resident would be a copy of his green card. Since you do not have a copy of that document, you will need to come up with some other secondary evidence. Try to think of a situation in which your husband had to produce his green card and whether or not he had to leave them a copy. For example, did he ever file for you to obtain your own green card, in which case he would have been required to reveal his own immigration status? Was he required to reveal his immigration status when you obtained the marriage license? If you absolutely cannot present primary or secondary evidence of his immigration status, the Immigration Officer will attempt to electronically verify his citizenship or immigration status from information contained in DHS computerized records. I would respond to USCIS and ask them to verify his records through their system.