A non-professional comment: I wouldn’t assume that a SS number is ever irrelevant. I won’t ask what you’ve done in past years, as it may not be applicable. Since the 1040 asks for a SS number specifically, and your husband has one, I would suggest you use it. If the two of you ever have joint accounts over $10, 000, you’ll have to list your husband as principal joint owner on an FBAR, and the FBAR requests a Tax Identification Number of the principal joint owner. I surely wouldn’t put NRA on an FBAR if there was a SS number available.
Of course, the two forms (FBAR and 1040) are not related, but you never know what tomorrow may bring (Pending reporting of accounts on Form1040 per the HIRE Act, or a claim for SS Retirement benefits).