Citizenship eligibility

Hello! I'm wondering if any of you can help me decipher the Naturalisation requirements.

My husband and I are Canadian citizens. His maternal grandmother (deceased) was an American Citizen and lived there most of her life. His mother, Canadian born and still living, never claimed her American Citizenship and therefore, none of her children are American citizens.

My question is this. are there any mechanisms under which my husband can apply for American citizenship?

He is eligible for a TN visa and we've lived in the US before under J1 and then H1 visas. We are now looking to go back.

I have to say I'm not sure, but obviously the grandmother can't do the paperwork. His mother might be able to get it but I'm not sure what the mechanism is if the relative is deceased, as sponsoring adult children requires an I-130 so it might be too late.

If his mother gets citizenship, she could sponsor him on an I-130 and then you would both get permanent residency.