Think about this way: before Viagra was invented not many man had sex in that age as your husband is now 64. So many interesting, smart, intellectual thing can replace sex ! :)And Viagra is not natural so I wont torture my husband if I was you. It is what it is!
The most likely cause is diabetic neuropathy, and he needs a complete diabetic screen immediately. Alpha-lipoic acid has some value in restoring damage due to this neuropathy.
He should see another urologist or at least go to his general physician or internist who may deal with this problem or refer him to another urologist. There may or may not be a solution to his ED, but as it stands now it has not been fully addressed nor explained.