Immigration question: Am I still attached to my ex-husband for immigration issues after divorce?

While you should have an experienced immigration attorney review your case to be sure, if you as a U.S. citizen spouse petitioned for your ex-husband to get his green card and signed an I-864 Affidavit of Support, then yes, it is possible that you are still financially "liable" or "responsible" for him and/or your stepson. If you have a copy of the I-864 that you filed in conjunction with his permanent residence application, you should review it to better understand the contract you made with the U.S. government in exchange for the immigration benefit(s) your ex-husband received based on your sponsorship. If you are legally divorced, then your marriage is terminated and immigration law doesn't "keep you married." It sounds as though perhaps your husband was admitted as a conditional permanent resident and perhaps he is trying to have the condition on the residence removed and is maybe seeking your help though if you are divorced, he will need to file a waiver by himself. DHS takes fraudulent marriages for immigration benefits seriously, and hopefully that is not the case here. However, it would be a good idea to consult with an attorney who is well-versed in family-based immigration if that is how your ex obtained his green card.