Proof of his US citizenship or his green card in addition to the documents you already mentioned and his W-2 attached to the 1040 tax return. Good luck.
You will need to show the joint sponsor is a US citizen or a green card holder.
(213) 394-4554 x0 Mr. Shusterman is a former INS Trial Attorney (1976-82) with over 35 years of immigration experience. His response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.
A separate I-864 in which the person indicates that s/he is a join sponsor.
NYC EXPERIENCED IMMIGRATION ATTORNEYS www.myattorneyusa.com; email: email@example.com; Phone: (866) 456-8654; Fax: 212-964-0440; Cell: 212-202-0325. The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter.
Your husband will need to send a letter explaining that he is unemployed and did not earn sufficient money to be required to file income tax returns.
Your joint sponsor will need to complete and sign an an additional I864 Affidavit of Support, send proof of US citizenship or LPR status, send a copy of his W2 or 1099 for the last year, and proof of current income.