1) How old was he when he applied for his GC before he got his GC?
2) Was he a US permanent resident before he turned 26?
Remember: The law says that all men living in the United States who are ages 18 through 25 must register, except those who are in valid non-immigrant status (e.g., students, visitors, and people who have been permitted to come to the US for limited periods of time, etc.)
it means All male US citizens, male US permanent residents, and male undocument aliens must register.
Tell your hubby to bring all non-immigrant status to the interview (write down the note of all dates from entry date to present. Each date should be stated with each immigrant status) and make copies of your passport including visas, entry dates, etc.
I think it is possible that it is required for men filed for 485 at ages between 18 and 25 because status was changed from valid non-immigrant status to AOS status
(AOS is not a non-immigrant status). so it could be possible that your hubby filed for 485 before he turned 26, and he was required to register. Do you have copies of 485 applications that he filed or copy of I-485 notice showing PD date or notice date or received date? Also show your hubby's green card. but I am not saying the information is correct. I could say it is possible. I am neither an expert nor a laywer. If the SSS rep said it, then trust the SSS.
Don't worry. Your hubby's age is already past 26. he cannot register it now because he is over 31. Past is past. Your hubby should show all documents. just bring everything with him (even he can drive U-haul truck that contains all documents to the interview.
I doubt Interview officer would ask for SS).